Solve for the Gammit(gamma +limit)

Calculus Level 3

lim x Γ ( 1 180 x ) Γ ( 1 1 180 x ) x = ? \large \lim_{x \to \infty} \frac {\Gamma \left(\frac 1{180x}\right)\Gamma \left(1- \frac 1{180x}\right)}x = \ ?

Notation: Γ ( ) \Gamma(\cdot) denotes the gamma function .


The answer is 180.

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3 solutions

We know that Γ ( 1 180 x ) Γ ( 1 1 180 x ) = π sin ( π 180 x ) \Gamma (\frac{1}{180x})\Gamma (1-\frac{1}{180x})=\dfrac {π}{\sin (\frac{π}{180x})} .

Changing x x to h = 1 x h=\frac{1}{x} , we get the required limit as

π π 180 lim h 0 π h 180 sin ( π h 180 ) \dfrac{π}{\frac{π}{180}}\displaystyle \lim_{h\to 0} \dfrac{\frac{πh}{180}}{\sin (\frac{πh}{180})}

= 180 × 1 = 180 =180\times 1=\boxed {180} .

That's a cool way.

Aruna Yumlembam - 1 year ago

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Thanks. You're from Manipur?

How did you guessed?

Aruna Yumlembam - 1 year ago

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I'm from WB, close to your place, isn't it? :D

Chew-Seong Cheong
May 21, 2020

Similar solution as @Alak Bhattacharya 's just with more details for the better understanding of some.

L = lim x Γ ( 1 180 x ) Γ ( 1 1 180 x ) x By Euler’s reflection equation: Γ ( z ) Γ ( 1 z ) = π sin ( π z ) = lim x π x sin π 180 x = lim x 180 π 180 x sin π 180 x Note that lim u 0 sin u u = 1 = 180 \begin{aligned} L & = \lim_{x \to \infty} \frac {\Gamma\left(\frac 1{180x}\right)\Gamma\left(1-\frac 1{180x}\right)}x & \small \blue{\text{By Euler's reflection equation: }\Gamma(z)\Gamma(1-z) = \frac \pi{\sin (\pi z)}} \\ & = \lim_{x \to \infty} \frac \pi{x \sin \frac \pi{180x}} \\ & = \lim_{x \to \infty} \frac {180 \cdot \blue{\frac \pi{180x}}}\blue{\sin \frac \pi{180x}} & \small \blue{\text{Note that }\lim_{u \to 0} \frac {\sin u}u = 1} \\ & = \boxed {180} \end{aligned}


Reference: Euler's reflection formula

Aruna Yumlembam
May 20, 2020

The first point to notice that we can use the reflection equation .Then we get π/sin(1/x) if we measure it in degrees.Then since limit as x approaches infinity sin(1/x)=π/(180x) .Then inputting it to the π/sin(1/x) yields gives 180x , dividing both side by x gives our answer as 180. (We can show that sin(1/x) is approximately π/(180x), when measured in degrees ,by Maclaurin series)

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