八下数学题第二大题求解答,(先化简再求值)
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八下数学题第二大题求解答,(先化简再求值)
![](http://img.wesiedu.com/upload/1/80/180492d57eae9cf87e3a21099df450fc.jpg)
![](http://img.wesiedu.com/upload/1/80/180492d57eae9cf87e3a21099df450fc.jpg)
![八下数学题第二大题求解答,(先化简再求值)](/uploads/image/z/9760478-14-8.jpg?t=%E5%85%AB%E4%B8%8B%E6%95%B0%E5%AD%A6%E9%A2%98%E7%AC%AC%E4%BA%8C%E5%A4%A7%E9%A2%98%E6%B1%82%E8%A7%A3%E7%AD%94%2C%EF%BC%88%E5%85%88%E5%8C%96%E7%AE%80%E5%86%8D%E6%B1%82%E5%80%BC%EF%BC%89)
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
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