sin(x+3п)cos(x-п)=2a+1
(-sinx)(-cosx)=2a+1
sinxcosx=2a+1
sin(2x)=4a+2
-1≤sin(2x)≤1
-1≤4a+2≤1
-3/4≤a≤-1/4
a的取值范围为[-3/4,-1/4]