Can you do Test 6 Section 3 #15?

Sure can. I’ll even do it two ways. First, I’ll plug in: let’s say a=3 and b=4. Then we can say that \left(a+\dfrac{b}{2}\right)^2=\left(3+\dfrac{4}{2}\right)^2=25. When you plug those values into the answer choices, which one gives you 25? Only choice D will: a^2+ab+\dfrac{b^2}{4}=3^2+(3)(4)+\dfrac{4^2}{4}=9+12+4=25.

To get this algebraically, all you need to do is FOIL. Just be super careful with the fraction.

    \begin{align*}&\left(a+\dfrac{b}{2}\right)\left(a+\dfrac{b}{2}\right)\\\\=&a^2+\dfrac{ab}{2}+\dfrac{ab}{2}+\left(\dfrac{b}{2}\right)^2\\\\=&a^2+\dfrac{2ab}{2}+\dfrac{b^2}{4}\\\\=&a^2+ab+\dfrac{b^2}{4}\end{align*}

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