若1/x+1/y=2,则2x-xy+2y/3x+5y+3y=怎么做呐吖??崩溃,教教我,过程详细点,感谢的!!
1/x+1/y=2 ﹙两边同乘以xy得﹚
x+y=2xy
﹙2x-xy+2y﹚/﹙3x+5xy+3y﹚
=[﹙2﹙x+y﹚-xy]/[3﹙x+y﹚+5xy]
=[2×2xy-xy]/[3×2xy+5xy]
=[4xy-xy]/[6xy+5xy]
=﹙3xy﹚/﹙11xy﹚
=3/11
﹙x+2y﹚﹙x-2y﹚-﹙2x-y﹚﹙-2x-y﹚
=﹙x+2y﹚﹙x-2y﹚+﹙2x-y﹚﹙2x+y﹚
=x²-4y²+4x²-y²
=5x²-5y²
=5﹙x+y﹚﹙x-y﹚
把x=8、y=-8代入上式得:
=5×[8+﹙-8﹚]×[8-﹙-8﹚]
=5×0×[8-﹙-8﹚]
=0
(2x-xy+2y)/(3x+5xy+3y)
【分子与分母都除以xy】
=(2/y-1+2/x)/(3/y+5+3/x)
【代入1/x+1/y=2】
=(2*2-1)/(3*2+5)
=3/11
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