Trigonometric limit 0 0 0^0

Calculus Level 4

Find the limit lim n ( sin n x + cos n x ) 1 n \displaystyle \lim_{n \to \infty}\left(\sin^n x+\cos^n x\right)^{\frac{1}{n}} at x = π 3 x=\frac \pi3 .

3 2 \frac {\sqrt 3}2 1 0 Does not exist 0.5

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3 solutions

bring out ( s i n x ) n (sinx)^n from the expression therefore we get

lim n \lim_{n \to \infty} s i n x ( 1 + ( c o t x ) n ) sinx(1+(cotx)^n) 1 n ^{\frac{1}{n}}

= [ ( lim n [(\lim_{n \to \infty} s i n x sinx ]*[ lim n \lim_{n \to \infty} ( 1 + ( c o t x ) n ) (1+(cotx)^n) 1 n ^{\frac{1}{n}} ]

As c o t ( π / 3 ) cot(\pi/3) is less than 1 therefore lim n \lim_{n \to \infty} ( 1 + ( c o t x ) n ) (1+(cotx)^n) 1 n ^{\frac{1}{n}} =1

therefore answer is s i n ( π / 3 ) \boxed{sin(\pi/3)} = 3 / 2 \sqrt{3}/2

In fact lim n ( a n + b n ) 1 n = m a x ( a , b ) \displaystyle\lim_{n\to\infty}(a^n+b^n)^{\frac 1n}=max(a,b)

展豪 張 - 5 years ago

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Yes, exactly

Nice question+solution..+1 .

Sabhrant Sachan - 5 years ago
Chew-Seong Cheong
May 14, 2018

L = lim n ( sin n x + cos n x ) 1 n = lim n sin x ( 1 + cot n x ) 1 n At x = π 3 = lim n 3 2 ( 1 + ( 1 3 ) n ) 1 n = 3 2 ( 1 + 0 ) 0 = 3 2 \begin{aligned} L & = \lim_{n \to \infty} \left(\sin^n x + \cos^n x \right)^\frac 1n \\ & = \lim_{n \to \infty} \sin x \left(1 + \cot^n x \right)^\frac 1n & \small \color{#3D99F6} \text{At }x = \frac \pi 3 \\ & = \lim_{n \to \infty} \frac {\sqrt 3}2 \left(1 + \left(\frac 1{\sqrt 3}\right)^n \right)^\frac 1n \\ & = \frac {\sqrt 3}2 \left(1 + 0 \right)^0 \\ & = \boxed{\dfrac {\sqrt 3}2} \end{aligned}

Robert Szafarczyk
May 16, 2018

You can plug in x = π 3 x=\frac {\pi}{3} straight away and from that it is pretty simple. But is plugging in straight away always correct (can it be deceitful) ?

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