I need to solve for t in the problem

-t^2 + 2t = 1/2

If I complete the square I get the correct answer 1-[sqrt2/2] or 1+[sqrt2/2]

if I use the quadratic formula [-b +/- (b^2 -4ac)/2a) I don't get the same answer. or I am doing something wrong.

Why is this? And how do I know which one to use so that I don't get the wrong answer. Thank you!