Determine the value of c for which the following quadratic equations

Question

will have one root.
x^2-5x+c=0

I have no idea what formula to use to figure this out.

Answer 1

look at the serd

b^2=4ac
25=4c
c= 25/4

when b^2>4ac, two roots, real
when b^2<4ac, two roots, complex
when b^2=4ac, one real root.

Answer 2

Bob
Well that was simple.
I couldn't find an example in my text like this. Thanks so much.