Question 751959
{{{
sin^4 (x)-cos^4 (x)=(sin^2 (x)-cos^2 (x))(sin^2 (x)+cos^2 (x))=(sin^2 (x)-cos^2 (x))(1)
}}}



and then


{{{
sin^2(x)-cos^2(x)=sin^2(x)-(1-sin^2(x))=2 sin^2 (x)-1
}}}


:)