I Smell Taylor Series

Algebra Level 3

Find sum of all real x x such that the below equation is true. e x = 1 + x + x 2 2 ! + x 3 3 ! + x 4 4 ! \large e^x = 1+x+\frac{x^2}{2!} + \frac{x^3}{3!} + \frac{x^4}{4!}


The answer is 0.

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1 solution

Maximos Stratis
Jun 9, 2017

For any x it is true that:
e x = 1 + x + x 2 2 ! + x 3 3 ! + x 4 4 ! + x 5 5 ! . . . e^{x}=1+x+\frac{x^2}{2!} + \frac{x^3}{3!} + \frac{x^4}{4!}+\frac{x^{5}}{5!}...
So for any x 0 x\neq 0 :
e x 1 + x + x 2 2 ! + x 3 3 ! + x 4 4 ! e^{x}\neq 1+x+\frac{x^2}{2!} + \frac{x^3}{3!} + \frac{x^4}{4!}
But for x = 0 x=0 all the extra terms become 0 0 so the equation holds true. Therefore, the sum of the roots is 0.

Note: I feel like i explained it poorly or maybe i've made a mistake. Please provide a better solution.

@maximos stratis Actually, you are on the right track.............we can see directly that the LHS is ALWAYS greater than the RHS except when x=0........so, that is the solution.......!!!

Aaghaz Mahajan - 2 years, 12 months ago

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I think it might be simpler to say that since that equation is true for all real numbers, every positive has a negative that cancels the two of them out when you sum, so the total is zero.

Tristan Goodman - 1 year, 11 months ago

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