f(b+x) = f( a+ b-a+x) = f(a- (b-a+x) ) = f (2a-b-x) = f( 2a-2b + b-x ) = f(b-x)
即 f( 2a-2b + u)= f(u),
f(x)是周期函数,周期 2b-2a