[tex]sin^2(x)cos^2(x)-cos^2(x)\\\\=sin^2(x)\cdot cos^2(x)-1\cdot cos^2(x)\\\\=cos^2(x)\cdot(sin^2(x)-1)\\\\=cos^2(x)\cdot[-(1-sin^2(x)]^{(*)}\\\\=cos^2(x)\cdot[-cos^2(x)]^{(*)}\\\\\boxed{=-cos^4(x)}\\------------------\\(*)\ used:\\\\sin^2(x)+cos^2(x)=1\to cos^2(x)=1-sin^2(x)[/tex]