设sinx+cosx=t ,sinx+cosx=√2sin(x+π/4)所以t属于[-√2,√2]t^2=1+2sinxcosx , sinxcosx=(t^2-1)/2 f(x）=（t^2-1）/[2(1+t)]=(t-1)/2属于[-(√2+1)/2,(√2-1)/2] 另外,分母不为零,所以1+sinx+cosx≠0 ,即t≠-1 ...