We study direct proportion and inverse proportion.

When two quantities are related to each other, their changes may follow certain patterns. We study two kinds.

When two quantities, x and y, vary in such a way that y/x is a constant, they are said to be in direct proportion, or y is said to be directly proportional to x.

We have y/x = k, where k is a constant. Or y = kx. On a drawing, it is a straight line passing through the origin.

For example, for a car traveling at a constant speed, the distance traveled  is directly proportional to the time taken. The constant here is known as the speed.

When two quantities, x and y, vary in such a way that xy is a constant, they are said to be in inverse proportion, or y is said to be inversely proportional to x.

We have xy = k, where k is a constant. Or y = k * 1/x. On a drawing, it is a hyperbola curve.

An example can be the speed and the time for a car travelling uniformly from A to B.

Homework:

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