Inequality Integration

Calculus Level 3

0 1 1 1 + x 8 d x \displaystyle\int_0^1 \dfrac{1}{1 + x^8} \, dx

Let L L denote the value of the integral above. Which of the answer choices must true?

π 4 < L < 1 \frac{\pi}{4} < L < 1 L = π 4 L = \frac{\pi}{4} None of these choices 0 < L < π 4 0 < L < \frac{\pi}{4} L > 1 L > 1

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2 solutions

Akhil Bansal
Oct 27, 2015

Since , x 2 > x 8 > 0 on ( 0 , 1 ) \large \text{Since}, x^2 > \color{#3D99F6}{ x^8} > 0 \ \text{on} \ (0,1)
x 2 + 1 > x 8 + 1 > 0 + 1 \large \Rightarrow x^2 + 1 > \color{#3D99F6}{x^8 +1 }> 0 + 1
1 1 + x 2 < 1 1 + x 8 < 1 \large \Rightarrow \dfrac{1}{1 + x^2} <\color{#3D99F6}{ \dfrac{1}{1 + x^8}} < 1
0 1 1 1 + x 2 dx = π 4 < 0 1 1 1 + x 8 dx < 1 = 0 1 1 dx \large \Rightarrow \displaystyle\int_0^1 \dfrac{1}{1 + x^2}\text{dx} = \color{#D61F06}{ \dfrac{\pi}{4}} < \color{#3D99F6}{\displaystyle\int_0^1 \dfrac{1}{1 + x^8}\text{dx}} < \color{#D61F06}{ 1} = \displaystyle\int_0^1 1 \cdot \text{dx}

Superb solution..

Sachin Anand - 5 years, 7 months ago
Lu Chee Ket
Dec 10, 2015

n=1: ln 2 0.6931 \ln 2 \approx 0.6931

n=2: π 4 0.7854 \frac{\pi}{4} \approx 0.7854

Example: 0. 1 8 = 0.00000001 0.1^8 = 0.00000001 which is much less than 0.1;

An effect of a tendency of zeroing x n x^n when n increased in a limit for 0 1 d x \int_0^1 d x = 1 - 0 = 1.

In fact, L = 0 1 1 1 + x 8 d x L = \displaystyle \int_0^1 \dfrac{1}{1 + x^8} \, dx \approx 0.92465

Answer: π 4 < L < 1 \boxed{\frac{\pi}{4} < L < 1}

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