Limited edition

Calculus Level 4

If a , b a,b and c c are constants, such that lim x 1 a x 2 + b x + c 3 ( x 1 ) 2 = 5 \displaystyle \lim_{x\to1} \dfrac{ \sqrt{ax^2+bx+c} - 3}{(x-1)^2} = 5 , find the value of a + c a+c .

Source: Bilkent University, 1st Midterm Question.


The answer is 69.

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1 solution

Haries Ajrun
Oct 28, 2014

For you who wondered what is the answer

Step 1 : Finding the value of a a + b b + c c

lim x 1 a x 2 + b x + c 3 ( x 1 ) 2 . lim x 1 ( x 1 ) 2 = 5.0 \lim _{ x\xrightarrow [ ]{ } 1 }{ \frac { \sqrt { a{ x }^{ 2 }+bx+c } -3 }{ { (x-1) }^{ 2 } } . } \lim _{ x\xrightarrow [ ]{ } 1 }{ { (x-1) }^{ 2 } } =5.0

a + b + c 3 = 0 \sqrt { a+b+c } -3=0

a + b + c = 9 a+b+c=9

Step 2 : Finding another equation

lim x 1 a x 2 + b x + c 3 ( x 1 ) 2 . lim x 1 ( x 1 ) = 5.0 \lim _{ x\xrightarrow [ ]{ } 1 }{ \frac { \sqrt { a{ x }^{ 2 }+bx+c } -3 }{ { (x-1) }^{ 2 } } . } \lim _{ x\xrightarrow [ ]{ } 1 }{ { (x-1) } } =5.0

lim x 1 ( a x 2 + b x + c 9 ( x 1 ) . 1 a x 2 + b x + c + 3 ) = 0 \lim _{ x\xrightarrow [ ]{ } 1 }{ (\frac { a{ x }^{ 2 }+bx+c-9 }{ { (x-1) } } . } \frac { 1 }{ \sqrt { a{ x }^{ 2 }+bx+c } +3 } )=0

lim x 1 ( a ( x 2 1 ) + b ( x 1 ) ( x 1 ) . 1 a + b + c + 3 ) = 0 \lim _{ x\xrightarrow [ ]{ } 1 }{ (\frac { a{ (x^{ 2 }-1) }+b(x-1) }{ { (x-1) } } . } \frac { 1 }{ \sqrt { a+b+c } +3 } )=0

1 6 lim x 1 ( a ( x + 1 ) + b ) = 0 \frac { 1 }{ 6 } \lim _{ x\xrightarrow [ ]{ } 1 }{ (a{ ({ x }+1) }+b) } =0

2 a + b 6 = 0 \frac { 2a+b }{ 6 } =0

2 a = b 2a=-b and it makes c = 9 + a c=9+a

Step 3 : Finding the value of a

lim x 1 a x 2 + b x + c 3 ( x 1 ) 2 = 5 \lim _{ x\xrightarrow [ ]{ } 1 }{ \frac { \sqrt { a{ x }^{ 2 }+bx+c } -3 }{ { (x-1) }^{ 2 } } } =5

lim x 1 ( a x 2 + b x + c 9 ( x 1 ) 2 . 1 a x 2 + b x + c + 3 ) = 5 \lim _{ x\xrightarrow [ ]{ } 1 }{ (\frac { a{ x }^{ 2 }+bx+c-9 }{ { (x-1) }^{ 2 } } . } \frac { 1 }{ \sqrt { a{ x }^{ 2 }+bx+c } +3 } )=5

lim x 1 a x 2 2 a x + a ( x 1 ) 2 . 1 a + b + c + 3 = 5 \lim _{ x\xrightarrow [ ]{ } 1 }{ \frac { a{ x }^{ 2 }-2ax+a }{ { (x-1) }^{ 2 } } . } \frac { 1 }{ \sqrt { a+b+c } +3 } =5

a 6 = 5 \frac { a }{ 6 } =5

a = 30 a = 30

now we can substitute a = 30 a = 30 to the equation c = 9 + a c=9+a we get c = 39 c=39 so the value of a + c = 69 a+c=69

Oh wow, I'm amazed that even though the numerator and denominator have different degrees, the limit can be finite.

Definitely worth a reshare.

Calvin Lin Staff - 6 years, 7 months ago

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YES, of course

Rostam Dana - 6 years, 7 months ago

The question is easy but what amazed me was the answer 69. Damn these university teachers are perverts !!. LOL

Arghyadeep Chatterjee - 3 years, 2 months ago

how about this bro? a = 20 b = 20 c = -31

Natsir Muhammad - 6 years, 7 months ago

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That limit would be undefined.

Calvin Lin Staff - 6 years, 6 months ago

same method!!! +1 :D

A Former Brilliant Member - 6 years, 2 months ago

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