f(x)=[sin(π/2－2x)]/[sin(π/4－x)]=[2sin(π/4－x)cos(π/4－x)]/[sin(π/4－x)]=2cos(x－π/4)
①定义域x－π/4≠kπ,即x≠kπ＋π/4,k∈Z；②增区间：2kπ－π≤x－π/4≤2kπ及定义域③sin2a=cos(2a－π/2)