y=2cosx/sin(45-x/2)
=4cosxcos(45-x/2)/[2sin(45-x/2)cos(45-x/2)]
=4cosxcos(45-x/2)/sin(90-x)
=4cosxcos(45-x/2)/cosx
=4cos(45-x/2)
=4cos(x/2-45)
=4cos（x/2-π/2)
所以当x/2-π/2∈（2kπ,π+2kπ),k∈Z时函数单调递减
即x∈(π+4kπ,3π+4kπ),k∈Z时函数单调递减
求函数y=2cosx/[sin(45-x/2)]的递减区间