Question

Let $f(x)=\sin x+2\cos x+3\tan x,$ using radian measure for the variable $x.$ In what interval does the smallest positive value of $x$ for which $f(x)=0$ lie?

$(0,1)$

$(1,2)$

$(2,3)$

$(3,4)$

$(4,5)$

Solution:

For $0\lt x\lt\dfrac{\pi}{2}$ all three terms are positive, so $f(x)\gt0.$ For $\dfrac{\pi}{2}\lt x\lt\pi,$ $\tan x$ is negative and dominates, keeping $f(x)\lt0.$ So no root occurs before $x=\pi.$

At $x=\pi,$ $f(\pi)=0+2(-1)+0=-2\lt0.$ At $x=\dfrac{5\pi}{4},$ $\tan x=1$ so $f=-\dfrac{\sqrt2}{2}+3\gt0.$ By the intermediate value theorem the smallest positive root lies in $\left(\pi,\dfrac{5\pi}{4}\right).$

Since $\pi\gt3$ and $\dfrac{5\pi}{4}\lt4,$ this interval sits inside $(3,4).$

Thus, the correct answer is D.

2017 AMC 12A Problem 15

Solution:

Problem 15 in Other Years