Trig+Logs

Algebra Level 3

sin ( ln x ) + 2 cos ( 3 ln x ) sin ( 2 ln x ) = 0 \sin(\ln x) + 2\cos(3\ln x)\sin (2\ln x) = 0

Find the smallest x > 1 x >1 satisfying the equation above.


The answer is 1.87.

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3 solutions

Dwaipayan Shikari
Jan 14, 2021

s i n ( l o g ( x ) ) + 2 c o s ( 3 l o g ( x ) ) s i n ( 2 l o g ( x ) ) = s i n ( l o g ( x ) ) + s i n ( 5 l o g ( x ) ) s i n ( l o g ( x ) ) \mathrm{sin(log(x))+2cos(3log(x))sin(2log(x))= sin(log(x))+ sin(5log(x))-sin(log(x))} = s i n ( 5 l o g ( x ) ) = 0 5 l o g ( x ) = k π =\mathrm{sin(5log(x))= 0\implies 5log(x)=kπ } where ( k Z ) (k∈Z) x = e k π 5 x=e^{\frac{kπ}{5}} Then solution is x = e π 5 = 1.8744 \color{#20A900}\boxed{x=e^{\frac{π}{5}}=1.8744} Here smallest possible n = 1 n=1 [as x > 1 x>1 and x 0 x≠0 ]

Chew-Seong Cheong
Jan 14, 2021

Let θ = ln x \theta = \ln x . Then we have:

sin θ + 2 cos 3 θ sin 2 θ = 0 sin θ + 2 ( 4 cos 3 θ 3 cos θ ) 2 sin θ cos θ = 0 Since θ = ln x > 0 sin θ 0 16 cos 4 θ 12 cos 2 θ + 1 = 0 Divide both sides by sin θ cos 2 θ = 3 ± 5 8 cos θ = ± 5 ± 1 4 θ = ln x = ( n + k 5 ) π where n Z , k = 1 , 2 , 3 , 4 \begin{aligned} \sin \theta + 2 \cos 3\theta \sin 2\theta & = 0 \\ \sin \theta + 2(4 \cos^3 \theta - 3\cos \theta) \cdot 2 \sin \theta \cos \theta & = 0 & \small \blue{\text{Since } \theta = \ln x > 0 \implies \sin \theta \ne 0} \\ 16 \cos^4 \theta - 12 \cos^2 \theta + 1 & = 0 & \small \blue{\text{Divide both sides by }\sin \theta} \\ \cos^2 \theta & = \frac {3\pm \sqrt 5}8 \\ \cos \theta & = \pm \frac {\sqrt 5 \pm 1}4 \\ \theta = \ln x & = \left(n + \frac k5\right)\pi & \small \blue{\text{where }n \in \mathbb Z, k = 1,2,3,4} \end{aligned}

For the smallest x > 1 x>1 , θ > ln 1 = 0 \implies \theta > \ln 1 = 0 or ( n + k 5 ) > 0 n = 0 , k = 1 \left(n+\dfrac k5\right)\pu > 0 \implies n = 0, k = 1 and ln x = π 5 x = e π 5 1.87 \ln x = \dfrac \pi 5 \implies x = e^\frac \pi 5 \approx \boxed{1.87} .

Rohan Joshi
Jan 14, 2021

pic of the solution

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