SSAT Upper Level Math : How to find the equation of a curve

Study concepts, example questions & explanations for SSAT Upper Level Math

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : How To Find The Equation Of A Curve

If the \(\displaystyle x\)-intercept of the line is \(\displaystyle 6\) and the slope is \(\displaystyle 1\), which of the following equations best satisfies this condition?

Possible Answers:

\(\displaystyle x-2y=-6\)

\(\displaystyle -2x-y=6\)

\(\displaystyle y=x+6\)

\(\displaystyle y=x-3\)

\(\displaystyle y=x-6\)

Correct answer:

\(\displaystyle y=x-6\)

Explanation:

Write the slope-intercept form.

\(\displaystyle y=mx+b\)

The point given the x-intercept of 6 is \(\displaystyle (6,0)\).

Substitute the point and the slope into the equation and solve for the y-intercept.

\(\displaystyle 0=(1)(6)+b\)

\(\displaystyle b=-6\)

Substitute the y-intercept back to the slope-intercept form to get your equation.

\(\displaystyle y=x-6\)

Example Question #2 : How To Find The Equation Of A Curve

A vertical parabola on the coordinate plane has vertex \(\displaystyle (-4, 10)\) and \(\displaystyle y\)-intercept \(\displaystyle (0, -2)\)

Give its equation.

Possible Answers:

\(\displaystyle y = \frac{3}{4} x^{2} + 6x+22\)

\(\displaystyle y = -\frac{3}{4} x^{2}- 6x-2\)

\(\displaystyle y = -\frac{1}{2} x^{2}- 4x+2\)

Insufficient information is given to determine the equation.

\(\displaystyle y = \frac{1}{2} x^{2}+4x+18\)

Correct answer:

\(\displaystyle y = -\frac{3}{4} x^{2}- 6x-2\)

Explanation:

The equation of a vertical parabola, in vertex form, is

\(\displaystyle y = a(x-h)^{2}+k\),

where \(\displaystyle (h,k)\) is the vertex. Set \(\displaystyle h = -4,k=10\):

\(\displaystyle y = a[x-(-4)]^{2}+10\)

\(\displaystyle y = a(x+4)^{2}+10\)

To find \(\displaystyle a\), use the \(\displaystyle y\)-intercept, setting \(\displaystyle x=0,y= -2\):

 \(\displaystyle -2 = a(0+4)^{2}+10\)

\(\displaystyle -2 = a\cdot 4^{2}+10\)

\(\displaystyle -2 = 16a +10\)

\(\displaystyle -12 = 16a\)

\(\displaystyle a =- \frac{3}{4}\)

The equation, in vertex form, is \(\displaystyle y =- \frac{3}{4}(x+4)^{2}+10\); in standard form:

\(\displaystyle y = -\frac{3}{4}(x+4)^{2}+10\)

\(\displaystyle y =- \frac{3}{4}(x^{2} + 8x+16)+10\)

\(\displaystyle y = -\frac{3}{4} x^{2}- 6x-12+10\)

\(\displaystyle y = -\frac{3}{4} x^{2}- 6x-2\)

Example Question #1 : How To Find The Equation Of A Curve

A vertical parabola on the coordinate plane has vertex \(\displaystyle (-3, -6)\); one of its \(\displaystyle x\)-intercepts is \(\displaystyle (-7, 0)\).

Give its equation.

Possible Answers:

\(\displaystyle y = 3x^{2}+36x+105\)

\(\displaystyle y =- \frac{3}{8}x^{2}- \frac{9}{4}x - \frac{75}{8}\)

\(\displaystyle y =- 3x^{2}-36x-111\)

Insufficient information is given to determine the equation.

\(\displaystyle y = \frac{3}{8}x^{2}+ \frac{9}{4}x - \frac{21}{8}\)

Correct answer:

\(\displaystyle y = \frac{3}{8}x^{2}+ \frac{9}{4}x - \frac{21}{8}\)

Explanation:

The equation of a vertical parabola, in vertex form, is

\(\displaystyle y = a(x-h)^{2}+k\),

where \(\displaystyle (h,k)\) is the vertex. Set \(\displaystyle h = -3,k=-6\):

\(\displaystyle y = a[x-(-3)]^{2}+ (-6)\)

\(\displaystyle y = a(x+3)^{2}-6\)

To find \(\displaystyle a\), use the known \(\displaystyle x\)-intercept, setting \(\displaystyle x= -7, y=0\):

\(\displaystyle 0 = a(-7+3)^{2}-6\)

\(\displaystyle 0 = a(-4)^{2}-6\)

\(\displaystyle 0 =16 a-6\)

\(\displaystyle 16a= 6\)

\(\displaystyle a = \frac{6}{16} = \frac{3}{8}\)

The equation, in vertex form, is \(\displaystyle y = \frac{3}{8}(x+3)^{2}-6\); in standard form:

\(\displaystyle y = \frac{3}{8}(x+3)^{2}-6\)

\(\displaystyle y = \frac{3}{8}(x^{2}+6x+9)-6\)

\(\displaystyle y = \frac{3}{8}x^{2}+ \frac{9}{4}x+ \frac{27}{8}-6\)

\(\displaystyle y = \frac{3}{8}x^{2}+ \frac{9}{4}x+ \frac{27}{8}- \frac{48}{8}\)

\(\displaystyle y = \frac{3}{8}x^{2}+ \frac{9}{4}x - \frac{21}{8}\)

Example Question #2 : How To Find The Equation Of A Curve

A vertical parabola on the coordinate plane has \(\displaystyle y\)-intercept \(\displaystyle (0,4)\); its only \(\displaystyle x\)-intercept is \(\displaystyle (6,0)\).

Give its equation.

Possible Answers:

\(\displaystyle y =- \frac{1}{6} x^{2}+\frac{1}{3}x+4\)

Insufficient information is given to determine the equation.

\(\displaystyle y =- \frac{1}{9} x^{2}+4\)

\(\displaystyle y = \frac{1}{6} x^{2}-\frac{5}{3}x+4\)

\(\displaystyle y = \frac{1}{9} x^{2}-\frac{4}{3}x+4\)

Correct answer:

\(\displaystyle y = \frac{1}{9} x^{2}-\frac{4}{3}x+4\)

Explanation:

If a vertical parabola has only one \(\displaystyle x\)-intercept, which here is \(\displaystyle (6,0)\), that point doubles as its vertex as well. 

The equation of a vertical parabola, in vertex form, is

\(\displaystyle y = a(x-h)^{2}+k\),

where \(\displaystyle (h,k)\) is the vertex. Set \(\displaystyle h = 6,k=0\):

\(\displaystyle y = a(x-6)^{2}+0\)

\(\displaystyle y = a(x-6)^{2}\)

To find \(\displaystyle a\), use the \(\displaystyle y\)-intercept, setting \(\displaystyle x= 0, y=4\):

\(\displaystyle 4= a(0-6)^{2}\)

\(\displaystyle 4= a( -6)^{2}\)

\(\displaystyle 4 = 36a\)

\(\displaystyle a = \frac{4}{36} = \frac{1}{9}\)

The equation, in vertex form, is \(\displaystyle y = \frac{1}{9}(x-6)^{2}\). In standard form:

\(\displaystyle y = \frac{1}{9}(x-6)^{2}\)

\(\displaystyle y = \frac{1}{9}(x^{2}-12+36)\)

\(\displaystyle y = \frac{1}{9} x^{2}-\frac{4}{3}x+4\)

Example Question #282 : Geometry

A vertical parabola on the coordinate plane has \(\displaystyle y\)-intercept \(\displaystyle (0, 6)\); one of its \(\displaystyle x\)-intercepts is \(\displaystyle (2,0)\).

Give its equation.

Possible Answers:

Insufficient information is given to determine the equation.

\(\displaystyle y =2 x^{2}-7 x + 6\)

\(\displaystyle y = x^{2}-5 x + 6\)

\(\displaystyle y = - x^{2}- x + 6\)

\(\displaystyle y = -2 x^{2}+ x + 6\)

Correct answer:

Insufficient information is given to determine the equation.

Explanation:

The equation of a vertical parabola, in standard form, is

\(\displaystyle y = ax^{2}+ bx + c\)

for some real \(\displaystyle a,b,c\)

\(\displaystyle c\) is the \(\displaystyle y\)-coordinate of the \(\displaystyle y\)-intercept, so \(\displaystyle c=6\), and the equation is

\(\displaystyle y = ax^{2}+ bx + 6\)

Set \(\displaystyle x=2, y=0\):

\(\displaystyle 0 = a\cdot 2^{2}+ b \cdot 2 + 6\)

\(\displaystyle 0 = 4a+2b + 6\)

However, no other information is given, so the values of \(\displaystyle a\) and \(\displaystyle b\) cannot be determined for certain. The correct response is that insufficient information is given.

Example Question #21 : X And Y Intercept

Ellipse 1

Give the equation of the above ellipse.

Possible Answers:

\(\displaystyle \frac{(x+6)^{2}}{16} + \frac{(y+2)^{2}}{64} = 1\)

\(\displaystyle \frac{(x-6)^{2}}{4} + \frac{(y-2)^{2}}{8} = 1\)

\(\displaystyle \frac{(x+6)^{2}}{4} + \frac{(y+2)^{2}}{8} = 1\)

\(\displaystyle \frac{(x-6)^{2}}{16} + \frac{(y-2)^{2}}{64} = 1\)

\(\displaystyle \frac{(x+6)^{2}}{64} + \frac{(y+2)^{2}}{256} = 1\)

Correct answer:

\(\displaystyle \frac{(x-6)^{2}}{16} + \frac{(y-2)^{2}}{64} = 1\)

Explanation:

The equation of the ellipse with center \(\displaystyle (h,k)\), horizontal axis of length \(\displaystyle 2a\), and vertical axis of length \(\displaystyle 2b\) is

\(\displaystyle \frac{(x-h)^{2}}{a^{2}} + \frac{(y-k)^{2}}{b^{2}} = 1\)

The ellipse has center \(\displaystyle (6,2)\), horizontal axis of length 8, and vertical axis of length 16. Therefore,

\(\displaystyle h = 6,k = 2\)\(\displaystyle a = \frac{8}{2} = 4\), and \(\displaystyle b = \frac{16}{2} = 8\).

The equation of the ellipse is

\(\displaystyle \frac{(x-6)^{2}}{4^{2}} + \frac{(y-2)^{2}}{8^{2}} = 1\)

\(\displaystyle \frac{(x-6)^{2}}{16} + \frac{(y-2)^{2}}{64} = 1\)

Example Question #1 : How To Find The Equation Of A Curve

Ellipse 1

Give the equation of the above ellipse.

Possible Answers:

\(\displaystyle \frac{(x+1)^{2}}{25} + \frac{(y+1)^{2}}{9} = 1\)

\(\displaystyle \frac{(x+1)^{2}}{100} + \frac{(y+1)^{2}}{36} = 1\)

\(\displaystyle \frac{(x-1)^{2}}{100} + \frac{(y-1)^{2}}{36} = 1\)

\(\displaystyle \frac{(x-1)^{2}}{25} + \frac{(y-1)^{2}}{9} = 1\)

\(\displaystyle \frac{(x-1)^{2}}{5} + \frac{(y-1)^{2}}{3} = 1\)

Correct answer:

\(\displaystyle \frac{(x-1)^{2}}{25} + \frac{(y-1)^{2}}{9} = 1\)

Explanation:

The equation of the ellipse with center \(\displaystyle (h,k)\), horizontal axis of length \(\displaystyle 2a\), and vertical axis of length \(\displaystyle 2b\) is

\(\displaystyle \frac{(x-h)^{2}}{a^{2}} + \frac{(y-k)^{2}}{b^{2}} = 1\)

The ellipse has center \(\displaystyle (1,1)\), horizontal axis of length 10, and vertical axis of length 6. Therefore,

\(\displaystyle h = k = 1\)\(\displaystyle a = \frac{10}{2} = 5\), and \(\displaystyle b = \frac{6}{2} = 3\).

The equation of the ellipse is

\(\displaystyle \frac{(x-1)^{2}}{5^{2}} + \frac{(y-1)^{2}}{3^{2}} = 1\)

\(\displaystyle \frac{(x-1)^{2}}{25} + \frac{(y-1)^{2}}{9} = 1\)

Example Question #22 : X And Y Intercept

Ellipse 1

Give the equation of the above ellipse.

Possible Answers:

\(\displaystyle \frac{(x+3)^{2}}{4} + \frac{(y+5)^{2}}{3} = 1\)

\(\displaystyle \frac{(x-3)^{2}}{16} + \frac{(y-5)^{2}}{9} = 1\)

\(\displaystyle \frac{(x+3)^{2}}{16} + \frac{(y+5)^{2}}{9} = 1\)

\(\displaystyle \frac{(x-3)^{2}}{4} + \frac{(y-5)^{2}}{3} = 1\)

\(\displaystyle \frac{(x-3)^{2}}{8} + \frac{(y-5)^{2}}{6} = 1\)

Correct answer:

\(\displaystyle \frac{(x-3)^{2}}{16} + \frac{(y-5)^{2}}{9} = 1\)

Explanation:

The equation of the ellipse with center \(\displaystyle (h,k)\), horizontal axis of length \(\displaystyle 2a\), and vertical axis of length \(\displaystyle 2b\) is

\(\displaystyle \frac{(x-h)^{2}}{a^{2}} + \frac{(y-k)^{2}}{b^{2}} = 1\)

The ellipse has center \(\displaystyle (3,5)\), horizontal axis of length 8, and vertical axis of length 6. Therefore,

\(\displaystyle h = 3,k = 5\)\(\displaystyle a = \frac{8}{2} = 4\), and \(\displaystyle b = \frac{6}{2} = 3\).

The equation of the ellipse is

\(\displaystyle \frac{(x-3)^{2}}{4^{2}} + \frac{(y-5)^{2}}{3^{2}} = 1\)

\(\displaystyle \frac{(x-3)^{2}}{16} + \frac{(y-5)^{2}}{9} = 1\)

 

Example Question #21 : X And Y Intercept

The \(\displaystyle y\)-intercept and the only \(\displaystyle x\)-intercept of a vertical parabola on the coordinate plane coincide with the \(\displaystyle y\)-intercept and the \(\displaystyle x\)-intercept of the line of the equation \(\displaystyle 3x+ 5y = 30\). Give the equation of the parabola.

Possible Answers:

Insufficient information is given to determine the equation.

\(\displaystyle y=\frac{5}{18}x^{2}- \frac{10}{3}x+10\)

\(\displaystyle y = \frac{3}{50} x^{2}-\frac{6}{5}x+6\)

\(\displaystyle y = \frac{18}{5} x^{2}-\frac{216}{5}x+ \frac{648}{5}\)

\(\displaystyle y = \frac{50}{3} x^{2}-\frac{1,000}{3}x+ \frac{5,000}{3}\)

Correct answer:

\(\displaystyle y = \frac{3}{50} x^{2}-\frac{6}{5}x+6\)

Explanation:

To find the \(\displaystyle y\)-intercept, that is, the point of intersection with the \(\displaystyle y\)-axis, of the line of equation \(\displaystyle 3x+ 5y = 30\), set \(\displaystyle x=0\) and solve for \(\displaystyle y\):

\(\displaystyle 3 (0)+ 5y = 30\)

\(\displaystyle 5y = 30\)

\(\displaystyle y = 6\)

The \(\displaystyle y\)-intercept is \(\displaystyle (0,6)\).

The \(\displaystyle x\)-intercept can be found by doing the opposite:

\(\displaystyle 3x+ 5 (0) = 30\)

\(\displaystyle 3x = 30\)

\(\displaystyle x=10\)

The \(\displaystyle x\)-intercept is \(\displaystyle (10,0)\).

The parabola has these intercepts as well. Also, since the vertical parabola has only one \(\displaystyle x\)-intercept, that point doubles as its vertex as well. 

The equation of a vertical parabola, in vertex form, is

\(\displaystyle y = a(x-h)^{2}+k\),

where \(\displaystyle (h,k)\) is the vertex. Set \(\displaystyle h = 10,k=0\):

\(\displaystyle y = a(x-10)^{2}+0\)

\(\displaystyle y = a(x-10)^{2}\)

for some real \(\displaystyle a\). To find it, use the \(\displaystyle y\)-intercept, setting \(\displaystyle x=0, y = 6\)

\(\displaystyle 6 = a(0-10)^{2}\)

\(\displaystyle 6 = a( -10)^{2}\)

\(\displaystyle 6 = 100a\)

\(\displaystyle a = \frac{6}{100} = \frac{3}{50}\)

The parabola has equation \(\displaystyle y =\frac{3}{50} (x-10)^{2}\), which is rewritten as

\(\displaystyle y =\frac{3}{50} (x^{2}-20x+100)\)

\(\displaystyle y = \frac{3}{50} x^{2}-\frac{6}{5}x+6\)

Example Question #231 : Coordinate Geometry

An ellipse on the coordinate plane has as its center the point \(\displaystyle (-4, 9)\). It passes through the points \(\displaystyle (-4, 5)\) and \(\displaystyle (11, 9)\). Give its equation.

Possible Answers:

\(\displaystyle \frac{(x-4)^{2}}{225} + \frac{(y+9)^{2}}{16} = 1\)

\(\displaystyle \frac{(x+4)^{2}}{225} + \frac{(y-9)^{2}}{16} = 1\)

\(\displaystyle \frac{(x+4)^{2}}{900} + \frac{(y-9)^{2}}{64} = 1\)

Insufficient information is given to determine the equation.

\(\displaystyle \frac{(x-4)^{2}}{900} + \frac{(y+9)^{2}}{64} = 1\)

Correct answer:

\(\displaystyle \frac{(x+4)^{2}}{225} + \frac{(y-9)^{2}}{16} = 1\)

Explanation:

The equation of the ellipse with center \(\displaystyle (h,k)\), horizontal axis of length \(\displaystyle 2a\), and vertical axis of length \(\displaystyle 2b\) is

\(\displaystyle \frac{(x-h)^{2}}{a^{2}} + \frac{(y-k)^{2}}{b^{2}} = 1\)

The center is \(\displaystyle (-4, 9)\), so \(\displaystyle h = -4\) and \(\displaystyle k = 9\).

To find \(\displaystyle a\), note that one endpoint of the horizontal axis is given by the point with the same \(\displaystyle y\)-coordinate through which it passes, namely, \(\displaystyle (11, 9)\). Half the length of this axis, which is \(\displaystyle a\), is the difference of the \(\displaystyle x\)-coordinates, so \(\displaystyle a = 11- (-4) = 15\). Similarly, to find \(\displaystyle b\), note that one endpoint of the vertical axis is given by the point with the same \(\displaystyle x\)-coordinate through which it passes, namely, \(\displaystyle (-4, 5)\). Half the length of this axis, which is \(\displaystyle b\), is the difference of the \(\displaystyle x\)-coordinates, so \(\displaystyle b = 9 - 5 = 4\).

The equation is 

\(\displaystyle \frac{(x-(-4))^{2}}{15^{2}} + \frac{(y-9)^{2}}{4^{2}} = 1\)

or 

\(\displaystyle \frac{(x+4)^{2}}{225} + \frac{(y-9)^{2}}{16} = 1\).

Learning Tools by Varsity Tutors