Complete the square than convert to vertex form. y=2x^2+8x-9

Question

asked Mar 22, 2021 129k views

1 Answer

Tom Alsberg · Answer 1 · 2021-03-27T05:45:40+0000

$y=2x^2+8x-9\\D=b^2-4ac\\D=64-4(2)(-9)\\D=64+72 > 0$

There are 2 roots so the only way to complete the square is,

$y=2x^2+8x-9\\y=2[(x^2+4x)]-9\\y=2[(x^2+4x+4)-4]-9\\y=2[(x+2)^2-4]-9\\y=2(x+2)^2-8-9\\y=2(x+2)^2-17$

Just factor 2 out of 2x^2+8x (just ignore the -9) then find the number that will make the terms be able to complete the square.

then complete the square and multiply 2 inside the brackets.

subtraction as you already get the vertex form and know how to complete the square.

Vertex Form:
y=2(x+2)^2-17