柠檬草的清香
幼苗
共回答了17个问题采纳率:100% 举报
由 f(xy)=f(x)+f(y),取x = y = 3,得:
f(xy) = f(9) = f(3) + f(3) = 2
由 f(xy)=f(x)+f(y),得:
f(xy) - f(x) = f(y)
令u = xy,v = x,则:
f(u) - f(v) = f(u/v)
由f(x)定义域(0,+∞),知,x > 0,x - 1 > 0
即:x > 1
f(x) > f(x-1) + 2
f(x) - f(x-1) > 2
f( x / (x-1) ) > f(9)
因为:f(x)是增函数,所以:
x / (x-1) > 9
同时 x > 0,x - 1 > 0
所以:x > 9(x-1)
解得:1 < x < 9/8
1年前
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