Question 1159430
 sin^2 x cos^2 x + cos^4 x = (1-sinx) (1+sinx)=(1-sin^2x)=cos^2x

left side factor out a cos^2x and get cos^2x(sin^2x+cos^2x)
but sin^2x+cos^2x=1
so cos^2 x (1)=cos^2 x

the key identities are cos^2x+sin^2 x=1
and 1-sin^2x=cos^2 x
and 1-cos^2 x= sin^2 x