(1)甲两次购买桑葚的平均单价Q①= (x+y)/2 元,
乙两次购买桑葚的平均单价Q②= 2xy/(x+y) 元.
(2)
(x+y)/2-2xy/(x+y)=[(x+y)²-4xy]/2(x+y)=(x-y)²/2(x+y)
∵x≠y,故(x-y)²/2(x+y)>0
∴(x+y)/2>2xy/(x+y)
故乙比较合算
(3)水果店从甲获得的利润:
(8-5)m+(6-5)m=3m+m=4m
水果店从乙获得的利润:
2m-5(m/8+m/6)=2m-5×7m/24=13m/24
总成本:5m×2+5(m/8+m/6)=275m/24
总利润:4m+13m/24=109m/24
利润率:
109m/(109+275)m×100%=38%