IIT JEE Problem too.

Calculus Level 3

T h e v a l u e o f l i m x 0 1 x 3 0 x t l n ( 1 + t ) t 4 + 4 d t = a b . F i n d t h e v a l u e o f a + b . The\quad value\quad of\quad \underset { x\rightarrow 0 }{ lim } \quad \frac { 1 }{ { x }^{ 3 } } \int _{ 0 }^{ x }{ \frac { t ln (1+t) }{ { { t } }^{ 4 } + 4 } } dt=\frac { a }{ b } .\\ Find\quad the\quad value\quad of\quad a+b.


The answer is 13.

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1 solution

Tanishq Varshney
Mar 23, 2015

since it is of 0 0 \frac{0}{0} form apply L-hospital and for integral part differentiation apply newton-leibnitz theorem

we get

lim x 0 x l n ( 1 + x ) 3 x 2 ( x 4 + 4 ) \displaystyle \lim_{x\to 0} \frac{x ln(1+x)}{3x^{2}(x^{4}+4)}

cancelling x x

lim x 0 l n ( 1 + x ) 3 x ( x 4 + 4 ) \displaystyle \lim_{x\to 0} \frac{ln(1+x)}{3x(x^4+4)}

lim x 0 x x 2 2 + x 3 3 . . . . . . . . 3 x ( x 4 + 4 ) \displaystyle \lim_{x\to 0} \frac{x-\frac{x^2}{2}+\frac{x^3}{3}-........}{3x(x^4+4)}

taking x x common from numerator

lim x 0 1 x 2 + x 2 3 . . . . . . . . 3 ( x 4 + 4 ) \displaystyle \lim_{x\to 0} \frac{1-\frac{x}{2}+\frac{x^2}{3}-........}{3(x^4+4)}

apply the limit x 0 x\to 0

u get 1 12 \frac{1}{12}

I wanna learn Calc. Where should I start?

Kumudesh Ghosh - 1 year ago

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