Let x = a/c > 0, y = b/a > 0, ∵a, b and c are all positive numbers, and both sides of the first inequality are divided by a at the same time.

Get 5/x-3≤y≤4/x- 1.

Clnb≥a+clnc, ∴lnb≥x+lnc∴ln b-lna ≥+ lnc-lna(e to the power of x), ∴lny≥ln(e to the power of x) -lnx

∴ lny ≥ ln (the x /x power of e), that is, the x /x power of y ≥ e.

Take the derivative of y=e to the power of x /x, and y' = e to the power of x (x- 1)/x? .

When x∑(0, 1), y is a decreasing function, and when x∑,