Lösning 2.1:3d

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 \qquad x^2 -10x +5^5 =x^2 -2\cdot 5x +5^5 =(x-5)^2