cos(π/4 -α)=3/5,-3π/2<α<-π/2,求cos(2α-π/4)

解：
*****************************************************
cos(π/4 -α)=cosπ/4*cosa+sinπ/4*sina=3/5
sina^2+cos^a2=1
-3π/2<α<-π/2
******************************************************
可以根据上式分别求出sin(π/4 -α);sina;cosa的值，将得出的值带入下式：

cos(2α-π/4)=cos[a-(π/4 -α)]
=cosa*cos(π/4 -α)+sina*sin(π/4 -α)
就可得出答案