| f(x)=−1(2‌sin‌2‌x−cos‌2‌x) As, we know that, if f(θ)=A‌sin‌θ+B‌cos‌θ Then, −√A2+B2≤f(θ)≤√A2+B2 Here, we have, f(x)=cos‌2‌x−2‌sin‌2‌x −√22+12≤f(x)≤√22+12 −√5≤f(x)≤√5 So, maximum value of f(x) is √5 .