Say you start with a general equation of x²+ax+b=0. (If the x² has a coefficient the you will need to divide by that first.)

You need to move to the equation (x+a/2)² -(a/2)²+b=0. Then put all of the constants together on the other side and square root everything so you end up with x-a/2 equalling plus or minus the square root of (a/2)²-b. You can then move to x equalling a/2 plus or minus the square root of (a/2)²-b.

For example, x²+6x+2=0 --> (x+3)²-3²+2 =0 --> (x+3)² =7 --> x+3 =+/-sqrt(7) --> x=-3+/-sqrt(7)

1i) solve x^2-4x-21=0

Related Maths GCSE answers

Write x^2 + 8x + 7 in the form (x + a)^2 + b

Factorise x^2 +7x +10

expand (2x+4)(x+2)

How do you complete the square?