Limit of Definite Integral #1

Calculus Level 5

I n = 0 1 x n a x + b d x \large I_n =\int_0^1 \frac{x^n}{ax+b} \, dx

Define the integral I n I_n as above for positive real variables a a and b b independent of x x and natural number n n .

lim n n I n = 1 λ a + μ b \large \lim_{n\to\infty} n I_n = \frac1{\lambda a + \mu b }

If λ \lambda and μ \mu are constants that satisfy the limit above, evaluate λ + μ \lambda + \mu .


The answer is 2.

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2 solutions

Satyajit Mohanty
Jul 8, 2015

Hassan Abdulla
Aug 13, 2019

I n = 0 1 x n a x + b d x = 1 n 0 1 t 1 n a t 1 n + b d t x = t 1 n , d x = 1 n t 1 n 1 n I n = 0 1 t 1 n a t 1 n + b d t a s n t 1 n 1 lim n n I n = 0 1 1 a + b d t = 1 a + b λ = 1 , μ = 1 \begin{aligned} I_n &= \int_0^1 \frac{x^n}{ax+b}dx = \frac{1}{n}\int_0^1 \frac{t^\frac{1}{n}}{at^\frac{1}{n}+b}dt && {\color{#D61F06} x =t^\frac{1}{n},dx=\frac{1}{n} t^{\frac{1}{n}-1} } \\ n I_n &= \int_0^1 \frac{t^\frac{1}{n}}{at^\frac{1}{n}+b}dt \\ & {\color{#D61F06} as \ n \rightarrow \infty \ t^\frac{1}{n} \rightarrow 1} \\ \lim \limits_{n \to \infty} n I_n &= \int_0^1 \frac{1}{a+b}dt = \frac{1}{a+b} \\ & {\color{magenta} \lambda =1 , \mu =1} \end{aligned}

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