f（x）=a[sinxcosπ/4+cosxsinπ/4]+b[sinxcosπ/4-cosxsinπ/4]
=√2a/2[sinx+cosx]+√2b/2[sinx-cosx]
f(-x)=√2a/2[sin(-x)+cos(-x)]+√2b/2[sin(-x)-cos(-x)]
=√2a/2[-sinx+cosx]+√2b/2[-sinx-cosx]
=√2a/2[-sinx+cosx]-√2b/2[sinx+cosx]
=f(x)
比较系数得：
√2a/2=-√2b/2
a=-b