If m:n=4, then m and n are proportional; If 5m=n, then m and n are proportional; If m/n= 1/2, then m and n are proportional; If nm=4, then m and n are inversely proportional; If m/9=4/n, then m and n are inversely proportional.

(1) ratio: two related quantities, one increases and the other increases. If the ratio (that is, quotient) of the two numbers corresponding to these two quantities is certain, these two quantities are called proportional quantities, and the relationship between them is called proportional relationship. Expressed in letters: If the letters X and Y are used to express two related quantities, their ratio is expressed as x÷y=k (certain).

(2) Inverse ratio: when one of the two related quantities changes, the other changes accordingly. If the product of two corresponding numbers is constant, these two quantities are called inverse proportional quantities, and the relationship between them is called inverse proportional relationship. Represented by letters: the two related quantities are "X" and "Y" respectively, and "K" is a constant.