Algebra 1 : How to multiply trinomials

Study concepts, example questions & explanations for Algebra 1

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Example Questions

Example Question #31 : Polynomials

Multiply:  \displaystyle (x^2+x-1)(x^2-2x-1)

Possible Answers:

\displaystyle x^4+x^3-4x^2-x+1

\displaystyle x^4+x^3-4x^2+x+1

\displaystyle x^4+x^3+4x^2-x+1

\displaystyle x^4-x^3-4x^2+x+1

\displaystyle x^4-x^3+4x^2+x+1

Correct answer:

\displaystyle x^4-x^3-4x^2+x+1

Explanation:

In order to solve, we will need to multiply each term of the first trinomial with all the terms of the second trinomial.  Sum all the terms together.

\displaystyle x^2(x^2-2x-1) = x^4-2x^3-x^2

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

\displaystyle -1(x^2-2x-1) = -x^2+2x+1

Add all of the terms and combine like terms.

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

The answer is:  \displaystyle x^4-x^3-4x^2+x+1

Example Question #32 : Polynomials

Multiply: \displaystyle (3x^2-2x+4)(x^2+x-5)

Possible Answers:

\displaystyle 3x^4+x^3-13x^2+14x-20

\displaystyle 3x^4-x^3-13x^2+14x-20

\displaystyle 3x^4-8x^3-17x^2+6x-20

\displaystyle 3x^4-x^3+13x^2-14x-20

\displaystyle 3x^4-5x^3-9x^2+6x-20

Correct answer:

\displaystyle 3x^4+x^3-13x^2+14x-20

Explanation:

Multiply each term of the first trinomial with the terms of the second trinomial.

\displaystyle 3x^2(x^2+x-5) = 3x^4+3x^3-15x^2

\displaystyle -2x(x^2+x-5) = -2x^3-2x^2+10x

\displaystyle 4(x^2+x-5) = 4x^2+4x-20

Combine like-terms.

\displaystyle 3x^4+3x^3-15x^2+(-2x^3-2x^2+10x)+(4x^2+4x-20)

The answer is: \displaystyle 3x^4+x^3-13x^2+14x-20

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