[1/（a平方-3a+2]+[1/（ a平方-5a+6）]+[1/(a平方-3a+12)]等于多少,快

[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)+1/(a-3)-1/(a-2)+1/(a-4)-1/(a-3)
=1/(a-4)-1/(a-1)
=[(a-1)-(a-4)]/(a-1)(a-4)
=3/(a²-5a+4)