For anyone who enjoys knowing why this sort of thing
works:
x2 =
x2 -5x+5x -52+52 =
x(x-5)+5(x-5)+52 =
(x-5)(x+5)+52 =
(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.