由于f( (a+b)/2 +x ) = f(a+(-a/2 + b/2 +x) )
=f(b -(-a/2 + b/2 +x)
=f( (a+b/2) -x)
即f( (a+b)/2 +x )=f( (a+b)/2 -x)
所对称轴是 x=(a+b)/2