y=2cos^2x-3=2cos^2x-1-2=cos4x-2
所以y=cos4x-2是偶函数
周期T=2π/4=π/2
单调递增区间为[π/4+kπ/2,π/2+kπ/2]
单调地减区间为[kπ/2,π/4+kπ/2]