Direct and Inverse Proportion

Direct & Inverse Proportion

Grade 8

Recapsulating Ratio and Proportion

• Ratio: https://www.youtube.com/watch?v=RQ2nYUBVvqI

• Proportion: https://www.youtube.com/watch?v=USmit5zUGas

Direct Proportion

• Two quantities x and y are said to be in direct proportion if whenever the value of x increases (or decreases), then the value of y increases or decreases in such a way that the ratio x:y i.e. x/y remains constant.

• When x and y are in direct proportion, then  $\frac{x_1}{y_1}=\frac{x_2}{y_2}=\frac{x_3}{y_3}=\frac{x_4}{y4},\ etc.\ and\ so\ on.$  

Indirect Proportion

• Two quantities x and y are said to be in indirect proportion if whenever the value of x increases or decreases then the value of y decreases or increases in such a way that xy remains constant.

• When x and y are in inverse proportion, then  $x_1\times y_1=x_2\times y_2=x_3\times y_3=x_4\times y_4,\ etc.\ and\ so\ on.$  

Direct Proportion

Indirect proportion

Sums

• A car covers 432 km in 36 litres of petrol. How much distance would it cover in 25 litres of Petrol?

• If 45 men can do a piece of work in 49 days, in how many day will 35 men do it?

• If x and Y are inversely proportional, find the value of x1, y1 and y2 $\frac{x}{y}\ \frac{12}{8}\ \frac{16}{y_1}\ \frac{x_1}{4}\ \frac{x_2}{16}\ \frac{48}{y_2}$  

• If 5 men or 7 women can earn $ 875 per day, how much would 10 men and 5 women earn per day?