Question

factorise
$4a {} ^{2} + 25b {}^{2} - 20ab$

Answer 1

this question on the basis

(a-b)^2=a^2 +b^2- 2 ×a×b

(2a)^2 +(5b)^2 - 2×2a ×5b

= (2a- 5b)^2