Integration sikhao!!!

This thread features my doubts in integration problems.....others can feel free to post their own!!!

OK HERE GOES:

I came across these questions in Brilliant Tutorials Elite Course ......

(I)
Both of the questions below have one or more options correct, but funnily enough i was able to pick out only one correct option in each question....

Pls give all possible correct options for both questions:

1)\int \frac{2x.dx}{1+x^4}

(a) tan-1(x2)+c (b)1/2 tan-1(x2)+c

(c)log(1+x4)+c (d) tan-1(1/x2)+c

2)\int \frac{dx}{x+\sqrt{x}}

(a)log(1+√x)+c (b) 1/2 log(1+√x)+c

(c)2log(1+√x)+c (d)-1/2 log(1+√x) +c

32 Answers

11
Mani Pal Singh ·

Strider koi question pooch
usse samajhenge[1]

1
skygirl ·

6)

(5-4x-x2)5/2 = (5+x)5/2(1-x)5/2

now write numerator as : 1/6[(5+x) + (1-x)]

then the integration bcmes:

1/6 [ ∫(5+x)/(5+x)5/2(1-x)5/2 dx + ∫(1-x)/(5+x)5/2(1-x)5/2 dx. ]

=1/6[ ∫dx/(5+x)3/2(1-x)5/2] + ∫dx/(5+x)5/2(1-x)3/2 ]

now apply the trick of maverick...

remeber that is a standard trick ...

11
Mani Pal Singh ·

8) put x =tanθ
and dx =sec2θdθ

and put y =sin-1p
and dy =1/√1-p2dp

naw change the limits and the question is done[1]

11
Anirudh Narayanan ·

PLEASE COMPARE THE QUESTIONS WITH JEE LEVEL OF TOUGHNESS AND THEN SOLVE:

#5)

(Subjective again)

\int \frac{(x-1)\; dx}{(x+1)\sqrt{x^3+x^2+x}}

#6)

\int \frac{dx}{\left(5-4x-x^2 \right)^{\frac{5}{2}}}

#7)

\int \frac{dx}{(x-1)^\frac{3}{4}(x+2)^\frac{5}{4}}}

#8)

Prove that

\int_{0}^{1}{\frac{tan^{-1}x\; dx}{x}}=\int_{0}^{\frac{\pi }{2}}{\frac{y\; dy}{siny}}

11
Anirudh Narayanan ·

[1]

Thanx da

So everytime, i get a question like this...... i'll remember u da. [1]

11
Mani Pal Singh ·

YUP ARAGON I THINK U GOT IT NOW

FOR UR INFORMATION U SOLVED A QUES OF IIT 1997 [1]

11
Anirudh Narayanan ·

that'll become......

\left[\sqrt{1+t^2} \right]^{2}_{0}

Hence....(√5-1)??

11
Mani Pal Singh ·

SOLVE THE FULL QUESTION
UWIONT GET ANYTHING FOR HALF SOLUTION[2]

DO IT THEN WE WILL CHECK[1]

11
Anirudh Narayanan ·

putting r/n---->t, dr/n----dt

It became
\int_{0}^{2}{\frac{t\; dt}{\sqrt{1+t^2}}}

Correct till now??

11
Mani Pal Singh ·

\lim_{n->infinity} 1/n\sum_{r=1}^{2n}{}r/\sqrt{n^{2}+r^{2}}

THIS ONE IS FOR ANIRUDH ONLY!!!!!!!!

11
Anirudh Narayanan ·

thanx in advance

11
Mani Pal Singh ·

yes i may say
in most of the other cases u have to struggle hard to find it
but it of premeir importance
chal i give u a ques 4 practice

11
Anirudh Narayanan ·

What should we look to do in such questions??

Always substitute r/n--->t??

11
Mani Pal Singh ·

yaar dekh iss mein
put r/n------>t
then 1/n-------->dt

this could be represented in the form of integral
lower limit and upper limit can be found out in the following method

For lower limit
sigma ke nicche waali term check kar
aggar woh constant hai to u put lower limit as 0

For upper limit
check the coefficient of n in the upper power of sigma
the coefficient must be written in the upper limit

so

noe the ques becomes

∫01x99dx

and hence 1/100
[1]

11
Anirudh Narayanan ·

Pls post complete solution with easy-to-understand reasoning

Exclude terminologies as much as possible

11
Anirudh Narayanan ·

OK then....

\lim_{n\rightarrow \infty}\frac{1}{n^{100}}\sum_{r=1}^{n}{r^{99}}

11
Anirudh Narayanan ·

(II)

Subjective type

(1)\int sin^3x\; cos(\frac{x}{2})dx

(2)\int sin^{-1}\left(\sqrt{\frac{x}{a+x}} \right)dx

(3)\int \frac{dx}{sinx(3+2cosx)}

(4)\int \frac{sinx\; dx}{sin3x}

11
Anirudh Narayanan ·

Pls give me the algorithm for converting summation to a definite integral...

Also pls illustrate with an example!

1
skygirl ·

[11] [11] [11] [11] [11] [11] [11]

11
Mani Pal Singh ·

2ND
UR ANSWER WAS SUPERIOR TO MINE JI(LADKION SE NO BEHASS )

1
skygirl ·

@manipal.... afetr where i left it .. in 2nd questions...

its jus one step...

becoz tanθsec2θdθ taken as second function... a child can integrate..

anyways... u have given another method... dat way its nice :)

1
skygirl ·

arey no no... all those partial fractions will become by parts !! [3] sorry..

1
skygirl ·

subj. 3)

∫dx/sinx(3+2cosx)

= ∫sinxdx/sin2x(3+2cosx)

= ∫sinxdx/(1-cos2x)(3+2cosx)

now put cosx=t...

11
Anirudh Narayanan ·

partial fractions??

GUESSED SO!![3] [3]

11
Mani Pal Singh ·

Q2
I THINK IT WOULD BE BETTER TO PUT
x+a=t2
dx=2tdt

upper niche waala t kat jaaega
so the ques becomes
∫sin-1t2-adt
this could be found out by PARTS (EDITED)[1]

1
skygirl ·

4) ∫sinx/sin3x = ∫sinx/sinx[3-sin2x] = ∫1/[2+cos2x]

=∫sec2xdx/[2sec2x+1]

=∫sec2xdx/[3+2tan2x]

tanx =t le lena ......

then it becomes simple quadratic..

1
skygirl ·

subjective:

2) sin-1x/(x+a)

take x=atan2θ => dx = 2atanθsec2θdθ

∫sin-1x/(x+a)

=∫sin-1(sinθ) .2atanθsec2θdθ

= 2a∫θtanθsec2θdθ

now do partial fractions taking θ as first function and tanθsec2θ as second function.

final ans : a[((x/a) +1)tan-1x/a - √x/a ]

1
°ღ•๓яυΠ·

subjctive tpe
3rd
tan(x/2)=t
proceed aajayega

ni aaya toh kck knck

1
vector ·

for 2nd q put x=a tan 2θ then u ll get 2aθtanθsec2θthen use by parts taking θ as 1st fn n rest as 2nd i think it ll do bt still better approaches invited

1
vector ·

for 1 subjective problem put sinx=2sinx/2 cosx/2 n then put cos x/2 as t

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