Question 17

A person standing at the center of an open ground first walks 32 meters towards the east, takes a right turn and walks 16 meters, takes another right turn and walks 8 meters, and so on. How far will the person be from the original starting point after an infinite number of such walks in this pattern?

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Let's draw the point on the coordinate axis. For the first point, we would go +32 on the x-axis. For the second point, we will go -16 on y asix.

For the third point, we go -8 on the x-axis. And for the next point, we go +4 on the y-axis. For the next one, we go +2 on the x-axis, and for the next one, we go -1 on the y-axis. For the next point, we go -1/2 on the x-axis, and for the next one, we go +1/4 on the y-axis.

Let's observe the moment wrt the x-axis.

+32, -8, +2, -1/2 ...

This is a G.P series with -1/4 as the standard ratio. 

The final point's x coordinate will be the sum of all the moments with respect to the x-axis. 

Sum to infinite terms of GP = $$\frac{a}{1-r}=\frac{32}{1-\left(-\frac{1}{4}\right)}=\frac{32}{\frac{5}{4}}=\frac{128}{5}$$

Similarly, let's see the moment wrt the y-axis.

-16, +4, -1, +1/4

This is a G.P series with -1/4 as the standard ratio.

The final point's y coordinate will be the sum of all the moments with respect to the y-axis.

Sum to infinite terms of GP = $$\frac{a}{1-r}=\frac{-16}{1-\left(-\frac{1}{4}\right)}=\frac{-16}{\frac{5}{4}}=\frac{-64}{5}$$

The distance from the origin to this point = $$\sqrt{\left(x_2-x_1\right)^2+\left(y_2-y_1\right)^2}$$ = $$\sqrt{\left(\frac{128}{5}-0\right)^2+\left(-\frac{64}{5}-0\right)^2}=\sqrt{\left(\frac{128}{5}\right)^2+\left(\frac{64}{5}\right)^2}$$

$$\sqrt{\left(\frac{64}{5}\right)^2\left(2^2+1^2\right)}=\sqrt{\left(\frac{64}{5}\right)^2\left(5\right)}$$

= $$\frac{64}{5}\sqrt{5}$$

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