上面是分子吧,应该是这样写：
1/1*5+1/5*9+...+1/45*49
=1/4[1-1/5]+1/4[1/5-1/9]+...+1/4[1/45-1/49]
=1/4[1-1/5+1/5-1/9+...+1/45-1/49]
=1/4*[1-1/49]
=1/4*48/49
=12/49