Question

How do you complete the square for `y=x^2 - 9x`?

Answer 1

`y=(x-9/2)^2-81/4`

Explanation:

`(x+a)^2=x^2+2ax+a^2` => `x^2+bx=(x+b/2)^2-b^2/4`
Therefore substituting `b=-9` we have:
`y=x^2-9x=(x-9/2)^2-81/4`