# CBSE Class 8 Math, Direct and Inverse Proportions

Class VIII Math
Notes for Direct and Inverse Proportions
FACTS THAT MATTER
If two quantities x and y vary (change) together in such a manner that the ratio of their corresponding values remains constant, then x and y are said to be in direct proportion.
if two quantities x and y vary (change) in such a manner that an increase in x causes a proportional decrease in y (and vice versa), then x and y are said to be inverse proportion.
If x and y are in a direct proportion, then
If x and y are in an inverse variation, then xy = constant.
WE KNOW THAT
The value of a variable is not constant and keeps on changing. There are many quantities whose value varies as per the circumstances. Some quantities have u relation with oilier quantities such that when one changes the other also changes. Such quantities are inter-redated. This is gilled a variation. Variation is of two types: (i) Direct variation and (ii) Inverse variation.
DIRECT PROPORTION
If two quantities are related in such a way that an increase in one quantity leads a corresponding increase in the other and vice versa, then this is a ease of direct variation. Also, a decrease in one quantity brings a corresponding decrease in the other.
Two quantities x and y are said to be in direct proportion, if

Note: I. In a direct proportion two quantities x and y vary with each other such that remains constant.
II. is always a positive number.
III. or k is called the constant of variation.