八下数学题第二大题求解答,（先化简再求值）

因为a+b=4, a-b=2
解得a=3, b=1
[(a^2-6ab+9b^2)/(a^2-2ab)]÷[(5b^2/(a-2b)-a-2b)-1/a]
=[(a-3b)²/a(a-2b)]÷[5b²/(a-2b)-(a²-4b²)/(a-2b)]-1/a
=[(a-3b)²/a(a-2b)]÷[(9b²-a²)/(a-2b)]-1/a
=[(a-3b)²/a(a-2b)]×[-(a-2b)]/(a+3b)(a-3b)-1/a
=-(a-3b)/a(a+3b)-1/a
=-(a-3b)/a(a+3b)-(a+3b)/a(a+3b)
=-2a/a(a+3b)
=-2/(a+3b)
=-2/(3+3)
=-1/3