a的2次方-4b的2次方+a+2b分之a的2次方-4b的2次方,其中a,b满足a-2b-2=0

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(a²-4b²)/[(a²-4b²)+(a+2b)]
=(a+2b)(a-2b)/(a+2b)(a-2b+1)
=(a-2b)/(a-2b+1)
=2/3