For anyone who enjoys knowing why this sort of thing

works:

x^{2} =

x^{2} -5x+5x -5^{2}+5^{2} =

x(x-5)+5(x-5)+5^{2} =

(x-5)(x+5)+5^{2} =

(x-5)(x+5)+25

...which is true for any value of x, but is easiest to calculate in your

head when x-5 and x+5 are

multiples of ten.