∵奇函数f(x),f(x+2)=-f(x)
∴f(7.5)
=-f(5.5)
=-[-f(3.5)]
=f(3.5)
=-f(1.5)
=-[-f(-0.5)]
=f(-0.5)
=-f(0.5)--奇函数的性质
=-0.5