命题“对于任意角\(θ\)，\(\cos ^{4}θ-\sin ^{4}θ=\cos 2θ\)”的证明过程为：“\(\cos ^{4}θ-\sin ^{4}θ=(\cos ^{2}θ-\sin ^{2}θ)(\cos ^{2}θ+\sin ^{2}θ)=\cos ^{2}θ-\sin ^{2}θ=\cos 2θ\)”，其应用了  \((\)    \()\)