PSAT Math : How to subtract trinomials

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #3 : Simplifying Polynomials

Evaluate the following:

\(\displaystyle (2x^2+\frac{3}{4}x-5) - (x^2+x-10)\)

Possible Answers:

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

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

\(\displaystyle x^2-\frac{7}{4}x + 10\)

\(\displaystyle x^2-\frac{1}{4}x -15\)

Correct answer:

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

Explanation:

To subtract these two trinomials, you first need to flip the sign on every term in the second trinomial, since it is being subtrated:

\(\displaystyle (2x^2+\frac{3}{4}x-5) - (x^2+x-10)\)

\(\displaystyle 2x^2+\frac{3}{4}x-5 - x^2-x+10\)

Next you can combine like terms. You have two terms with \(\displaystyle x^2\), two terms with \(\displaystyle x\), and two terms with no variable:

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

Example Question #1 : How To Subtract Trinomials

Find the difference:

\(\displaystyle (25a^3+14a^2+9a)-(-6a^3-14a^2+8a)\)

Possible Answers:

\(\displaystyle 31a^3+a\)

\(\displaystyle 19a^3+a\)

\(\displaystyle 31a^3+28a^2+a\)

\(\displaystyle 19a^3+17a\)

\(\displaystyle 31a^3+14a^2+a\)

Correct answer:

\(\displaystyle 31a^3+28a^2+a\)

Explanation:

Find the difference:

\(\displaystyle (25a^3+14a^2+9a)-(-6a^3-14a^2+8a)\)

Distribute the negative to the second trinomial:

\(\displaystyle 25a^2+14a^2+9a+6a^3+14a^2-8a\)

Combine like terms:

\(\displaystyle 31a^3+28a^2+a\)

Example Question #1 : How To Subtract Trinomials

Subtract:

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

Possible Answers:

\(\displaystyle 2x^2+20x+6\)

\(\displaystyle x^4+20x^2+6\)

\(\displaystyle 20x\)

\(\displaystyle 20x+6\)

\(\displaystyle 20x^2\)

Correct answer:

\(\displaystyle 20x\)

Explanation:

When subtracting trinomials, first distribute the negative sign to the expression being subtracted, and then remove the parentheses: \(\displaystyle (x^2+15x-3)-(x^2-5x-3)=x^2+15x-3-x^2+5x+3\)

Next, identify and group the like terms in order to combine them: \(\displaystyle (x^2-x^2)+(15x+5x)+(3-3)=20x\).

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