[1/（a平方-3a+2]+[1/（ a平方-5a+6）]+[1/(a平方-7a+12)] 最后一个3换成7=1/(a-1)(a-2)+1/(a-2)(a-3)+1/(a-3)(a-4)=[(a-1)-(a-2)]/(a-1)(a-2)+[(a-2)-(a-3)]/(a-2)(a-3)+[(a-3)-(a-4)]/(a-3)(a-4)=1/(a-2)-1/(a-1)+...