A trigonometric equation and a polynomial

Geometry Level 4

Find the number of solutions to the given equation

sin x = x 10 . \sin x = \dfrac{x}{10}.


The answer is 7.

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2 solutions

Kushal Bose
Jan 6, 2017

As sin x 1 \sin x \leq 1 so there will be solution if x ! 0 1 x 10 = 3 π + ϵ \dfrac{x}{!0} \leq 1 \implies x \leq 10=3 \pi +\epsilon .It will give solution upto 3 π 3 \pi .After 3 π 3 \pi sine curve will go negative and x / 10 x/10 will remain positive.This line will cut every concave part twice.Between 3 π 3 \pi there are three concave part where two parts are above X-axis and one is below X-axis.It will cut only positive X-axis.So, there will be No.of concave part × no. of cuts = 2 × 2 = 4 \text{No.of concave part} \times \text{no. of cuts}=2 \times 2=4 solutions .As sine is symmetric then for the negative part there will be another four solutions.Total 4 + 4 = 8 4+4=8 solutions.But we have calculated x = 0 x=0 twice.Then number of distinct solutions are 8 1 = 7 8-1=7 solutions.

Draw the graphs of y=sinx and y=x/10 .Easily you can obtain 7 solutions

i was thinking the same too but you cannot exactly sketch it

A Former Brilliant Member - 4 years, 4 months ago

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Yes,it took me some time to figure out about the 7th intersection.

A Former Brilliant Member - 4 years, 4 months ago

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