f(x)=2sinxcosx-asinx-acosx
令sinx+cosx=t=(√2)sin(x+π/4)
则-√2≤t≤√2
f(x)=t^2-1-at=t^2-at-1=(t-a/2)^2-1-a^2/4
1）当-√2≤a/2≤√2,即-2√2≤a≤2√2时
t=a/2时取到最小值-1-a^2/4
2）当a/2>√2即a>2√2时,t=√2时取到最小值1-√2a
3）当a/2