ACT Math : Algebra

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #14 : Polynomials

Solve the equation \(\displaystyle x^{2} - 25x = 0\)

Possible Answers:

\(\displaystyle 10\)

\(\displaystyle 20\)

\(\displaystyle 25\)

\(\displaystyle -5\)

\(\displaystyle 5\)

Correct answer:

\(\displaystyle 25\)

Explanation:

To answer this question, we are solving for the values of \(\displaystyle x\) that make this equation true.

To this, we need to get \(\displaystyle x\) on a side by itself so we can evaluate it. To do this, we first add \(\displaystyle 25x\) to both sides so that we can then begin to deal with the \(\displaystyle x^{2}\) value. So, for this data:

\(\displaystyle x^{2}-25x=0\rightarrow x^{2}=25x\)

\(\displaystyle x^{2}\)can also be written as \(\displaystyle x\cdot x\). Therefore:

\(\displaystyle x^{2}=25x\rightarrow x\cdot x=25x\)

Now we can divide both sides by \(\displaystyle x\) and find the value of \(\displaystyle x\).

\(\displaystyle \frac{x\cdot x}{x}=\frac{25x}{x}\rightarrow x=25\)

Therefore, the answer to this question is \(\displaystyle x=25\)

Example Question #1 : How To Add Polynomials

Simplify the following expression. 

\(\displaystyle (2x^3+6x^2-3x+1) + (x^3-2x^2+4x-3)\)

Possible Answers:

\(\displaystyle 3x^3+2x^2+2x+2\)

\(\displaystyle 2x^3+6x^2+2x+2\)

\(\displaystyle 3x^3+3x^2+x+4\)

\(\displaystyle 3x^3+4x^2+x-2\)

\(\displaystyle 3x^4-4x^2+7x-2\)

Correct answer:

\(\displaystyle 3x^3+4x^2+x-2\)

Explanation:

Line up each expression vertically. Then combine like terms to solve. 

\(\displaystyle 2x^3+6x^2-3x+1\)

\(\displaystyle x^3-2x^2+4x-3\)

____________________

\(\displaystyle 2x^3+x^3 = 3x^3\)

\(\displaystyle 6x^2-2x^2 =4x^2\)

\(\displaystyle -3x+4x=x\)

\(\displaystyle 1-3=-2\)

 

Thus, the final solution is \(\displaystyle 3x^3+4x^2+x-2\).

Example Question #11 : Polynomial Operations

What is the value of \(\displaystyle x\) when \(\displaystyle 5x-5y=5y-5x\)

Possible Answers:

\(\displaystyle x+y=0\)

\(\displaystyle x-y=0\)

\(\displaystyle 25x=y\)

\(\displaystyle 25y=x\)

\(\displaystyle x=y\)

Correct answer:

\(\displaystyle x=y\)

Explanation:

\(\displaystyle 5x-5y=5y-5x\)

In adding \(\displaystyle 5x\) to both sides:

\(\displaystyle 5x-5y+5x=5y-5x+5x\)

\(\displaystyle 10x-5y=5y\)

. . .and adding \(\displaystyle 5y\) to both sides:

\(\displaystyle 10x-5y+5y=5y+5y\)

. . .the variables are isolated to become:

 \(\displaystyle 10x=10y\)

After dividing both sides by \(\displaystyle 10\), the equation becomes:

\(\displaystyle x=y\)

Example Question #2 : How To Add Polynomials

Add the following polynomials:

\(\displaystyle (5x^{3}+31x^{2}-17x-6)+(-2x^{3}+9x^{2}+34x-12)\)

Possible Answers:

\(\displaystyle 3x^{3} + 40x^{2} +17x -18\)

\(\displaystyle -3x^{3} + 40x^{2} -17x +6\)

\(\displaystyle 3x^{3} + 22x^{2} +10x -18\)

\(\displaystyle -7x^{3} + 40x^{2} +17x -18\)

\(\displaystyle 7x^{3} + 40x^{2} +17x +6\)

Correct answer:

\(\displaystyle 3x^{3} + 40x^{2} +17x -18\)

Explanation:

This is a problem where elimination can be help you save a little time. You can eliminate options quickly by simplifying one power at a time and comparing your work with the answer choices.

To begin, reorder the problem so that all like terms are next to each other. When doing so, keep an eye on your signs so that you don't accidentally make a mistake.

\(\displaystyle (5x^{3}-2x^{3}) + (31x^{2}+9x^{2}) +(-17x+34x) +(-6-12)\)

From here, combine each pair of terms. As you do so, compare your work with the answer choices.

\(\displaystyle (5x^{3}-2x^{3})=3x^{3} }\)   Eliminate any answer choices that have a different \(\displaystyle x^3\) term.

\(\displaystyle (31x^{2}+9x^{2})=40x^2\) Eliminate any answer choices that have a different \(\displaystyle x^2\) term.

\(\displaystyle (-17x+34x)=17x\) Eliminate any answer choices that have a different x term.

\(\displaystyle (-6-12)= -6+-12=-18\) Eliminate any answer choices that have a different constant term.

Once you put all of your solutions together, the correct answer looks like this:

\(\displaystyle 3x^{3} + 40x^{2} +17x -18\)

Example Question #1 : Binomials

What is the value of the coefficient in front of the term that includes \(\displaystyle x^{2}y^{7}\) in the expansion of \(\displaystyle \left ( 2x-y \right )^{9}\)?

 

Possible Answers:

\(\displaystyle -36\)

\(\displaystyle -144\)

\(\displaystyle 36\)

\(\displaystyle 144\)

Correct answer:

\(\displaystyle -144\)

Explanation:

Using the binomial theorem, the term containing the x2 ywill be equal to

 Act_math_113_14(2x)2(–y)7

=36(–4x2 y7)= -144x2y7

Example Question #2 : Binomials

A function of the form \(\displaystyle f(x)=ax^2+b\) passes through the points \(\displaystyle (0,7)\) and \(\displaystyle (-2,19)\).  What is the value of \(\displaystyle a\) ?

Possible Answers:

\(\displaystyle 7\)

\(\displaystyle -3\)

\(\displaystyle 2\)

\(\displaystyle -2\)

\(\displaystyle 3\)

Correct answer:

\(\displaystyle 3\)

Explanation:

The easisest way to solve for \(\displaystyle a\) is to begin by plugging each pair of coordinates into the function.

Using our first point, we will plug in \(\displaystyle 0\) for \(\displaystyle x\) and \(\displaystyle 7\) for \(\displaystyle f(x)\).  This gives us the equation

\(\displaystyle 7=a(0)^2+b\).

Squaring 0 gives us 0, and multiplying this by \(\displaystyle a\) still gives 0, leaving only \(\displaystyle b\) on the right side, such that

\(\displaystyle 7 = b\).

We now know the value of \(\displaystyle b\), and we can use this to help us find \(\displaystyle a\).  Substituting our second set of coordinates into the function, we get

\(\displaystyle 19 = a(-2)^2+b\)

 which simplifies to

\(\displaystyle 19=4a+b\).

However, since we know \(\displaystyle b=7\), we can substitute to get

\(\displaystyle 19=4a+7\)

subtracting 7 from both sides gives

\(\displaystyle 12 = 4a\)

and dividing by 4 gives our answer

\(\displaystyle 3 = a\).

Example Question #14 : Polynomials

\(\displaystyle 2x^{2} \cdot x^{3}y^{2} \cdot 3y\) is equivalent to which of the following?

Possible Answers:

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

\(\displaystyle 6xy\)

\(\displaystyle 5x^{5}y^{3}\)

\(\displaystyle 6x^{6}y^{2}\)

\(\displaystyle 6x^{5}y^{3}\)

Correct answer:

\(\displaystyle 6x^{5}y^{3}\)

Explanation:

To answer this problem, we need to multiply the expressions together, being mindful of how to correctly multiply like variables with exponents. To do this, we add the exponents together if the the like variables are being multiplied and subtract the exponents if the variables are being divided. So, for the presented data:

\(\displaystyle 2x^{2} \cdot x^{3}y^{2} \cdot 3y = 2x^{5} \cdot 3y^{3}\)

We then multiply the remaining expressions together. When we do this, we will multiply the coefficients together and combine the different variables into the final expression. Therefore:

\(\displaystyle 2x^{5} \cdot 3y^{3} = 6x^{5}y^{3}\)

This means our answer is \(\displaystyle 6x^{5}y^{3}\).

Example Question #2 : How To Find The Value Of The Coefficient

Give the coefficient of \(\displaystyle x^{2}\) in the product  

\(\displaystyle \left ( x+ 0.4\right ) (x - 0.2) (3x-0.7)\).

Possible Answers:

\(\displaystyle 1.3\)

\(\displaystyle 0.5\)

\(\displaystyle -0.1\)

\(\displaystyle 0.7\)

\(\displaystyle 2.5\)

Correct answer:

\(\displaystyle -0.1\)

Explanation:

While this problem can be answered by multiplying the three binomials, it is not necessary. There are three ways to multiply one term from each binomial such that two \(\displaystyle x\) terms and one constant are multiplied; find the three products and add them, as follows:

 

\(\displaystyle \left (\underline{ x}+ 0.4\right ) (\underline{x} - 0.2) (3x\underline{-0.7})\)

\(\displaystyle x \cdot x \cdot (-0.7) = -0.7x^{2}\)

 

\(\displaystyle \left (\underline{ x}+ 0.4\right ) (x \underline{-0.2})(\underline{3x}-0.7)\)

\(\displaystyle x \cdot (-0.2) \cdot 3x= -0.6x^{2}\)

 

\(\displaystyle \left (x+ \underline{0.4}\right ) (\underline{x} - 0.2)(\underline{3x}-0.7)\)

\(\displaystyle 0.4 \cdot x \cdot 3x = 1.2 x^{2}\)

 

Add: \(\displaystyle -0.7x^{2}+ (-0.6x^{2})+ 1.2x^{2} = -0.1x^{2}\).

The correct response is \(\displaystyle -0.1\).

Example Question #1 : How To Find The Value Of The Coefficient

Give the coefficient of \(\displaystyle x^{2}\) in the product  

\(\displaystyle \left ( 2x + \frac{1}{3}\right ) \left ( x- \frac{1}{6}\right ) \left ( x+ \frac{1}{4}\right )\)

Possible Answers:

\(\displaystyle \frac{11}{12}\)

\(\displaystyle \frac{1}{4}\)

\(\displaystyle \frac{1}{2}\)

\(\displaystyle \frac{1}{6}\)

\(\displaystyle \frac{7}{6}\)

Correct answer:

\(\displaystyle \frac{1}{2}\)

Explanation:

While this problem can be answered by multiplying the three binomials, it is not necessary. There are three ways to multiply one term from each binomial such that two \(\displaystyle x\) terms and one constant are multiplied; find the three products and add them, as follows:

\(\displaystyle \left ( \underline{2x }+ \frac{1}{3}\right ) \left ( \underline{x}- \frac{1}{6}\right ) \left ( x\underline{+ \frac{1}{4}}\right )\)

\(\displaystyle 2x \cdot x \cdot \frac{1}{4} = \frac{1}{2}x^{2}\)

 

\(\displaystyle \left ( \underline{2x }+ \frac{1}{3}\right ) \left ( x \underline{- \frac{1}{6}}\right ) \left ( \underline{x}+ \frac{1}{4}\right )\)

\(\displaystyle 2x \cdot \left (- \frac{1}{6} \right ) \cdot x = - \frac{1}{3}x^{2}\)

 

\(\displaystyle \left ( 2x\underline{ + \frac{1}{3}}\right ) \left ( \underline{x}- \frac{1}{6}\right ) \left ( \underline{x}+ \frac{1}{4}\right )\)

\(\displaystyle \frac{1}{3} \cdot x \cdot x= \frac{1}{3}x^{2}\)

 

Add: \(\displaystyle \frac{1}{2}x^{2} +\left ( - \frac{1}{3}x^{2} \right ) + \frac{1}{3}x^{2} = \frac{1}{2}x^{2}\)

The correct response is \(\displaystyle \frac{1}{2}\).

Example Question #4 : Binomials

Give the coefficient of \(\displaystyle x^{5}\) in the binomial expansion of \(\displaystyle \left ( 2x+ 0.5\right )^{8}\).

Possible Answers:

\(\displaystyle 1\)

\(\displaystyle 1,680\)

\(\displaystyle 224\)

\(\displaystyle 14\)

\(\displaystyle 26,880\)

Correct answer:

\(\displaystyle 224\)

Explanation:

If the expression \(\displaystyle \left ( A x + B\right )^{n}\) is expanded, then by the binomial theorem, the \(\displaystyle x^{k}\) term is

\(\displaystyle C(n, k) \cdot\left ( Ax \right )^{k} \cdot B ^{n - k}\)

\(\displaystyle = C(n, k) \cdot A ^{k} \cdot B ^{n - k} \cdot x^{k}\)

or, equivalently, the coefficient of \(\displaystyle x^{k}\) is 

\(\displaystyle C(n, k) \cdot A ^{k} \cdot B ^{n - k}\)

Therefore, the \(\displaystyle x^{5}\) coefficient can be determined by setting 

\(\displaystyle A = 2, B =0.5, k=5, n = 8\):

\(\displaystyle C(8,5) \cdot 2^{5} \cdot 0.5 ^{8-5}\)

\(\displaystyle =C(8,5) \cdot 2^{5} \cdot 0.5 ^{3}\)

\(\displaystyle = 56\cdot 32 \cdot 0.125\)

\(\displaystyle =224\)

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