Question

Hi, I am looking for the x coordinates only..f(x)= -x^2 + 2x + 15


Thanks..

Answer 1

factor it and solve ;

what i do is ignore the negative in front of the x^2 for now

giving you x^2 + 2x +15

take 1x from the x^2 and multiply it with the 15 giving you 15x. now figure out which to numbers multiply to make 15 (there is only 1 x 15, and 3 x 5) out of these two which can be added to make a 2 (because of the 2x)your answer would be 3 and 5 if one was a negative but we wont put a negative in front of them because the origional equation had the x^2 as a negative so that changes the problem a little.

so now you should have a factor out equation which is (x+ 5 and -x+ 3) one of the x is negative because the x^2 was negative now just put both equation equal to zero and solve for x you should get;

x= -5, and 3 now put it back into original polynomial and whichever one doesnt equal zero wont be an answer

Answer 2

You can't just ignore one - sign. It's easier to have the leading coefficient be positive, so multiply the whole equation by -1 to get

x^2 - 2x - 15 = 0
(x-5)(x+3) = 0

so, x = 5,-3