Evaluate the limit.
L = x → 0 lim x ( x + 4 ) 2 3 + e x − 9
Bonus : Solve this limit without using L'hopital's rule .
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L = x → 0 lim x ( x + 4 ) 2 3 + e x − 9 = x → 0 lim x 8 ( 1 + 4 x ) 2 3 + e x − 9 Using Maclaurin series = x → 0 lim x 8 ( 1 + 8 3 x + 1 2 8 3 x 2 − 1 0 2 4 x 3 − . . . ) + ( 1 + x + 2 x 2 + 6 x 3 + . . . ) − 9 = x → 0 lim x ( 8 + 3 x + 1 6 3 x 2 − 1 2 8 x 3 − . . . ) + ( 1 + x + 2 x 2 + 6 x 3 + . . . ) − 9 = x → 0 lim x 4 x + 1 6 1 1 x 2 + 3 8 4 6 1 x 3 + . . . = x → 0 lim 4 + 1 6 1 1 x + 3 8 4 6 1 x 2 + . . . = 4
Nice approach ...
Substituting x = 0 , L takes the form of 0 0 , the derivative of L is 2 3 × ( x + 4 ) 1 / 2 . By L'Hopital's Rule substituting the derivative of L with x = 0 we get the required limit as 4
You were not supposed to use L Hopitals rule.
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That's a bonus question and this is the solution for the actual question.
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First , transformed into an algebraic limit
Now using standard following theorems ,
x → 0 lim x e x − 1 = 1 , x → a lim x − a x n − a n = n a n − 1
We get ,
L = x → 0 lim x ( x + 4 ) 3 / 2 + e x − 9 = x → 0 lim x ( x + 4 ) 3 / 2 − 8 + x e x − 1 = x → 0 lim ( x + 4 ) − 4 ( x + 4 ) 3 / 2 − 8 + 1 = t → 4 lim t − 4 t 3 / 2 − 4 3 / 2 + 1 (by putting t = x + 4 ) = 2 3 ⋅ 4 1 / 2 + 1 = 4
This approach is without L Hosp. rule. View @Svatejas Shivakumar 's solution for the bonus one.