先化简,再求值:(1- a+1分之1)-a的平方+2a+1分之a,其中a=根号3-1

原式=[ 1- 1/(a+1) ]- [ a/(a²+2a+1) ]
=1 - 1/(a+1) - a/(a+1)²
=(a+1)²/(a+1)² - (a+1)/(a+1)² -a/(a+1)²
=[ (a+1)²-(a+1) -a ]/(a+1)²
=(a²+2a+1-a-1-a)/(a+1)²
=a²/(a+1)²
当a=√3-1时
原式=(√3-1)²/(√3-1+1)²
=(√3²-2√3+1²)/√3²
=(3-2√3+1)/3
=(4-2√3)/3