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Garrett Black

Garrett Black

Answered question

2022-06-28

I have the following expression:
0 L T x 2 2 × 10 4 e 2 × 10 4 x d x + L T L T 2 2 × 10 4 e 2 × 10 4 x d x = 7065080

Answer & Explanation

Sawyer Day

Sawyer Day

Beginner2022-06-29Added 30 answers

I wonder if there is or not a mistake (the L 2 must disappear).
I 1 = 1 5000 0 L x 2 e x / 5000 d x = 5 × 10 7 e L / 5000 ( 5 × 10 7 + 10 4 L + L 2 )
I 2 = L 2 5000 L e x / 5000 d x = e L / 5000 L 2
(1) f ( L ) = I 1 + I 2 7065080 = 42934920 10 4 e L / 5000 ( L + 5000 ) = 0
The only explicit solution is given in terms of Lambert function
L = 5000 ( 1 + W 1 ( 1073373 1250000 e ) ) = 3289.36
What is amuzing is that expanding ( 1 ) as a Taylor series around L = 0
f ( L ) = 7065080 + L 2 L 3 7500 + L 4 100000000 + O ( L 5 )
and using series reversion
L = t + t 2 15000 + 11 t 3 1800000000 + O ( t 4 ) where t = f ( L ) + 7065080
Since we want f ( L ) = 0, then the approximation
L 176627 375 + 46942897 2250000 176627 10 = 3243.79

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