f(x+2)=-f(x)
则-f(x+2)=f(x)
所以f(x+4)
=f[(x+2)+2]
=-f(x+2)
=f(x)
f(7.5)
=f(4+3.5)
=f(3.5)
=f(4-0.5)
=f(-0.5)
奇函数
=-f(0.5)
0<0.5<1,所以f(0.5)=0.5
所以f(7.5)=-0.5