答案是9分之5怎么算的告诉我方法！=（1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)。。。。。（1-1/9)(1+1/9)=1/2x3/2x2/3x4/3x3/4x5/4x。。。。。。x8/9x10/9(前后约分）=1/2x10/9=5/9