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Ask iit jee aieee pet cbse icse state board community Community Discussion Question: limits
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shikhar.jain50 (47)

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evaluate

    
pink_ele (1380)

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dis becomes
sec^-1{1/1+(1/x)}
put x=infinity
sec^-1(1)=0

nobody is wrong
even a stopped clock is right twice a day
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Aka (32)

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I Am Not Sure of the Answer


let x=1/t


then limit changes to lim t=>0 sec^-1 (1/(1+t))


now let  t=cos(theta)


then sec^-1 (1/1+cos(theta))


==>> sec^-1 (1/2cos^2(theta/2))


==>> sec^-1 (sec^2(theta/2)/2)


==>> [((theta/2)/2)*sec (theta/2)]/2


now (theta)=(cos^-1 (t))/2


==>>{[ cos^-1 (t)/2 ] *1/[cos(cos^-1 (t)]}/2


==>>{ (cos^-1(t)/2) *1/[t/2] }/2


now put t=0 you will get Zero


You can also try with LN Hospital rule its very easy with it


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if not understand i can give you better explation then this


 

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fireoftheforest (2)

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=sec^-1 (infinity/infinity)=sec^-1(infinity)=pi/2
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fireoftheforest (2)

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rate me if useful
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ananth910 (38)

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i say that since sec^-1(infinity)=pi/2,3pi/2,.....
hence the value of the limit is not unique hence limit does not exist.

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ananth
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mukulaish (238)

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wats the answer shikhar??
my answer is coming out to be zero.. the approach is same as pink_ele....


post the answer
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oneyeartogo (217)

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I think question is incorrect. For positive values of x, numerator is less than denominator ( i.e it is a fraction) for which function is not defined

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ashish040191 (142)

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arre yaar bracket se x bahar nikal lene par sec-(1/1+1/x) bachta hai. since x tends to infinity we get sec-1 i.e 0 is the answer

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oneyeartogo (217)

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Thanks for clearing y doubt


 

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shikhar.jain50 (47)

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my dear friends

option for given question are

0

pie

pie/2

none

and correct answer is none

it seems ananth910 is correct but has failed to give satisfactory explanation


and my dear friend oneyeartogo the question is absolutely right

 

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animal (615)

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yes i m also getting the ans as 0


tell me if i m correct


also @oneyeartogo when x tends to infinity x tends to x+1 so we get sec inverse 1 which is 0.


but i still think that ur doubt is genuine and a good thought provoking one .I might be wrong but i have given my view on this.


hope u got it...

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oneyeartogo (217)

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@animal you are absolutely right.




 


I thought the same way. As x ----->infinity the function just comes into existense or as you put it x ----> x+1

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thedumbheadwithnobrain (887)

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Since,x\rightarrow\infty,x>0\\Now,for\;every\;x>0,x+1>x\\hence,\left(\frac{x}{x+1}\right)<1\\sec^{-1}(any\;number\;less\;than\;1)=not\;defined\\So,answer\;is\;not\;defined\\\\Going\;by\;above\;methods\\let\;x=\frac{1}{t}\\\lim_{t\to0}sec^{-1}\left(\frac{1}{1+t}\right)\;is\;the\;limit\;to\;be\;evaluated\\RHL\\\lim_{h\to0}sec^{-1}\left(\frac{1}{1+h}\right)=not\;defined...\left(as\;\frac{1}{1+h}<1\right)\\LHL\\\lim_{h\to0}sec^{-1}\left(\frac{1}{1-h}\right)=sec^{-1}\left(\frac{1}{1-(\rightarrow0)}\right)=sec^{-1}(1)=0\\Since,LHL\neq RHL\\Hence,Limit\;does\;not\;exist.

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oneyeartogo (217)

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@thedumbheadwithnobrain          


I don't think your solution is right. Your doubt is same as mine (answered above)


Look at line number 9



Since,x\rightarrow\infty,x>0\\Now,for\;every\;x>0,x+1>x\\hence,\left(\frac{x}{x+1}\right)<1\\sec^{-1}(any\;number\;less\;than\;1)=not\;defined\\So,answer\;is\;not\;defined\\\\Going\;by\;above\;methods\\let\;x=\frac{1}{t}\\\lim_{t\to0}sec^{-1}\left(\frac{1}{1+t}\right)\;is\;the\;limit\;to\;be\;evaluated\\RHL\\\lim_{h\to0}sec^{-1}\left(\frac{1}{1+h}\right)=not\;defined...\left(as\;\frac{1}{1+h}<1\right)\\LHL\\\lim_{h\to0}sec^{-1}\left(\frac{1}{1-h}\right)=sec^{-1}\left(\frac{1}{1-(\rightarrow0)}\right)=sec^{-1}(1)=0\\Since,LHL\neq RHL\\Hence,Limit\;does\;not\;exist.