Question

Factorise it
x4 -39x2+25

Answer 1

x4+x2+25=(x2)2+52+x2=(x2+5)2−2(x2)(5)+x2=(x2+5)2−10x2+x2=(x2+5)2−9x2=(x2+5)2−(3x)2=(x2+3x+5)(x2−3x+5)