Example

Completing the Square for x2+6xx^2 + 6x

Complete the square for the expression x2+6xx^2 + 6x by finding the constant that transforms it into a perfect square trinomial.

Step 1 — Choose the correct form of the Binomial Squares Pattern. Because the two terms are connected by addition, use the form a2+2ab+b2=(a+b)2a^2 + 2ab + b^2 = (a + b)^2.

Step 2 — Identify aa. The first term x2x^2 matches a2a^2, so a=xa = x.

Step 3 — Find bb from the middle term. The middle term in the pattern is 2ab2ab. Setting 2ab=6x2ab = 6x and substituting a=xa = x gives 2xb=6x2 \cdot x \cdot b = 6x, so b=126=3b = \frac{1}{2} \cdot 6 = 3.

Step 4 — Square bb to find the missing last term. The last term of the perfect square trinomial is b2=32=9b^2 = 3^2 = 9.

Step 5 — Write the completed trinomial and factor.

x2+6x+9=(x+3)2x^2 + 6x + 9 = (x + 3)^2

Adding 99 to x2+6xx^2 + 6x completes the square. The key calculation is taking half the coefficient of the linear term (126=3\frac{1}{2} \cdot 6 = 3) and squaring it (32=93^2 = 9) to determine the constant. Once the constant is added, the resulting trinomial matches the Binomial Squares Pattern and factors directly into (x+3)2(x + 3)^2.

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Updated 2026-04-21

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