Completing Squares
It's given the following equation:
[tex]x^2-20x=-2x-80[/tex]We are required to express the equation in the form:
[tex](x+a)^2=b[/tex]The first step is sending all the variables to the left side of the equation.
Adding 2x:
[tex]\begin{gathered} x^2-20x+2x=-80 \\ \\ \text{Simplifying:} \\ x^2-18x=-80 \end{gathered}[/tex]To complete squares, we need to recall the following identity:
[tex]p^2+2pq+q^2=(p+q)^2[/tex]The expression on the left side is missing the third term to be a perfect square. Note that comparing
p=x
2pq = -18x
This means that
q = -18x/2p
q = -18x/2x
q = -9
Now we know the value of the second term, we need to add q^2=81:
[tex]x^2-18x+81=-80+81[/tex]The left side of the equation is the square of x-9, and the right side can be calculated:
[tex](x-9)^2=1[/tex]Now we have the required expression, where a=-9 and b = 1
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