f(7.5)=f(4+3.5)=f(3.5)
f(3.5)=f(4+(-0.5))=f(-0.5)
f(x)是奇函数,f(x)=-f(-x)
f(-0.5)=-f(0.5)
x=0.5在0≤x≤1上
f(0.5)=0.5
-f(0.5)=-0.5