y=cos²x－sinxcosx=1/2(1+cos2x)-1/2sin2x=1/2cos2x-1/2sin2x+1/2=√2/2(√2/2*cos2x-√2/2*sin2x)+1/2=√2/2cos(2x+π/4)+1/2∴当2x+π/4=2kπ+π/2,k∈Z时,y取得最大值(√2+1)/2当2x+π/4=2kπ-π/2,k∈Z时,y...