Question

 3. 25у^2-а^2= с^2+4bc+4b^2= 

Answer 1

$25y^2-a^2=(5y)^2-a^2=(5y-a)(5y+a)$
$c^2+2bc+4b^2=c^2+2*c*(2b)+(2b)^2=(c+2b)^2$