what is the value of c to make this expression a perfect square trinomial?

x^2-15x+c

1 answer

take 1/2 of the coefficient of the middle term, then square it

i.e.

1/2(15) = 15/2
now we square this: (15/2)^2 = 225/4 or 56.25 in decimals

c = 225/4