Question

Draw the graph of xy = 20, x, y > 0. Use the graph to find y when x = 5, and to find x when y = 10

Answer 1

$\begin{array}{ccccc}x& 1& 2& 4& 5\\ y& 20& 10& 5& 4\end{array}$

From the table, we observe that as x increases y decrease. This type of variation is called indirect variation.

$y\propto \frac{1}{x}$ or xy = k where k is a constant of proportionality. We find that,

1 × 20 = 2 × 10 = 4 × 5 = 20 = k

We get k =20.

Plot the points (1,20), (2,20), (4,5),(5,4)

x-axis 1 cm = 2 units; y-axis 1 cm = 2 units

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From the graph and also from table, it is clear that when x is 5 the corresponding value on y –axis is 4

And when y is 10 the corresponding value on x – axis is 2.