This is the limit 2

Calculus Level 5

lim x 0 a sin ( x ) b x + c x 2 + x 3 2 x 2 ln ( 1 + x ) 2 x 3 + x 4 \large{\displaystyle \lim_{x\to 0} \frac{a\sin (x)-bx+cx^{2}+x^{3}}{2x^{2} \ln (1+x)-2x^3+x^{4}}}

If the given limit exists and is finite for constants a , b , c a,b,c , then it is equal to d e \frac{d}{e} where d , e d,e are coprime natural numbers.

Evaluate a + b + c + d + e a+b+c+d+e .


The answer is 55.

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1 solution

Adrian Peasey
Jun 28, 2015

Consider the McLaurin expansion ln ( 1 + x ) = x x 2 2 + x 3 3 + \ln(1+x)=x-\frac{x^2}{2}+\frac{x^3}{3}+\ldots

Taking the first two terms alone results in the denominator of the limit being 0 for all x x , so the third term is also required. As such we will want to take the McLaurin expansion of the a sin ( x ) a\sin(x) term to the 5th order.

This gives a new fraction of:

a ( x x 3 6 + x 5 120 ) b x + c x 2 + x 3 2 x 2 ( x x 2 2 + x 3 3 ) 2 x 3 + x 4 \frac{a(x-\frac{x^3}{6}+\frac{x^5}{120})-bx+cx^2+x^3}{2x^2(x-\frac{x^2}{2}+\frac{x^3}{3})-2x^3+x^4}

Grouping powers and cancelling we get:

( a b ) x + c x 2 + ( 1 a 6 ) x 3 + a x 5 120 2 x 5 3 \frac{(a-b)x+cx^2+(1-\frac{a}{6})x^3+\frac{ax^5}{120}}{\frac{2x^5}{3}}

Splitting this gives:

( a b ) x 2 x 5 3 + c x 2 2 x 5 3 + ( 1 a 6 ) x 3 2 x 5 3 + a x 5 120 2 x 5 3 = 3 ( a b ) 2 x 4 + 3 c 2 x 3 + ( 3 a 2 ) 2 x 2 + 3 a 240 \frac{(a-b)x}{\frac{2x^5}{3}}+\frac{cx^2}{\frac{2x^5}{3}}+\frac{(1-\frac{a}{6})x^3}{\frac{2x^5}{3}}+\frac{\frac{ax^5}{120}}{\frac{2x^5}{3}}=\frac{3(a-b)}{2x^4}+\frac{3c}{2x^3}+\frac{(3-\frac{a}{2})}{2x^2}+\frac{3a}{240}

In order for the limit to exist, the numerator of all terms with x x in the denominator must be 0, which means a = b a=b , c = 0 c=0 , and a = 6 a=6 .

The limit is therefore 3 × 6 240 = 3 40 \frac{3\times6}{240}=\frac{3}{40} which means x + y + a + b + c = 3 + 40 + 6 + 6 + 0 = 55 x+y+a+b+c=3+40+6+6+0=\boxed{55}

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