Question 754631
{{{
(cosx-secx)^2=(cos(x)-1/(cos(x)))^2=cos^2(x)-2cos(x)(1/cos(x))+1/cos^2(x)=cos^2(x)-2+sec^2(x)
}}}


which we rewrite as


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