- Two quantities x and y are said to be in direct proportion if they increase or decrease together in such a manner that the ratio of their corresponding values remains constant.
- That is, x y = k where k is a positive number, if x and y are in direct proportion or vary directly.
- In case of direct proportion, if y1, y2 are the values of y corresponding to the values x1, x2 of x respectively, then 1 2 1 2 x x y y.
- Two quantities x and y are said to be in inverse proportion if an increase in x causes a proportional decrease in y and vice-versa, in such a manner that the product of their corresponding values remains constant.
- That is, xy = k where k is a positive number, if x and y are in inverse proportion. In this case, if y1, y2 are the values of y corresponding to the values x1, x2 of x respectively, then x1 y1= x2 y2 or x y x y1 1 2 2
- Quantities increasing or decreasing together need not always be in direct proportion, same in the case of inverse proportion.
- When two quantities x and y are in direct proportion (or vary directly), they are written as x ∝ y. Symbol “∝” stands for ‘is proportional to’.
- When two quantities x and y are in inverse proportion (or vary inversely) they are written as x ∝ 1 y
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