T/2=5/4-1/4T=2ω=2π/T=2π/2=πf(x)=cos(πx+φ)过(1/4,0)cos(π/4+φ)=0π/4+φ=π/2φ=π/4——这就是φf(x)=cos(πx+π/4)2kπ<πx+π/4<π+2kπ2k2k-1/4单调递减区间：(2k-1/4,2k+3/4) (k∈Z)
