From Förberedande kurs i matematik 1

By writing the expression as \displaystyle x^2 -2\cdot 5x +5^5 , applying the squaring rule \displaystyle x^2-2ax +a^2=(x-a)^2 then gives

\displaystyle x^2 -10x + 25 = x^2 -2\cdot 5x +5^5 = (x-5)^2\,\textrm{.}