a的平方+2a=4a²+2a+1=5(a+1)²=5a+1分之1-a的平方-1分之1除以a的平方-2a+1分之a+1=1/(a+1)-1/(a+1)(a-1)×(a-1)²/(a+1)=1/(a+1)-(a-1)/(a+1)²=(a+1-a+1)/(a+1)²=2/(a+1)²∵(a+1)²...