If the given limit exists and is finite for constants , then it is equal to where are coprime natural numbers.
Evaluate .
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Consider the McLaurin expansion ln ( 1 + x ) = x − 2 x 2 + 3 x 3 + …
Taking the first two terms alone results in the denominator of the limit being 0 for all x , so the third term is also required. As such we will want to take the McLaurin expansion of the a sin ( x ) term to the 5th order.
This gives a new fraction of:
2 x 2 ( x − 2 x 2 + 3 x 3 ) − 2 x 3 + x 4 a ( x − 6 x 3 + 1 2 0 x 5 ) − b x + c x 2 + x 3
Grouping powers and cancelling we get:
3 2 x 5 ( a − b ) x + c x 2 + ( 1 − 6 a ) x 3 + 1 2 0 a x 5
Splitting this gives:
3 2 x 5 ( a − b ) x + 3 2 x 5 c x 2 + 3 2 x 5 ( 1 − 6 a ) x 3 + 3 2 x 5 1 2 0 a x 5 = 2 x 4 3 ( a − b ) + 2 x 3 3 c + 2 x 2 ( 3 − 2 a ) + 2 4 0 3 a
In order for the limit to exist, the numerator of all terms with x in the denominator must be 0, which means a = b , c = 0 , and a = 6 .
The limit is therefore 2 4 0 3 × 6 = 4 0 3 which means x + y + a + b + c = 3 + 4 0 + 6 + 6 + 0 = 5 5