The sum of the series 12!+14!+16!+ is:

Question

The sum of the series $\dfrac{1}{2!}+\dfrac{1}{4!}+\dfrac{1}{6!}+\cdots$ is:

Accepted Answer

We know that

$$e = 1 + {1 \over {1!}} + {1 \over {2!}} + {1 \over {3!}} + ..........$$

and

$${e^{ - 1}} = 1 - {1 \over {1!}} + {1 \over {2!}} - {1 \over {3!}} + .........$$

$$\therefore$$ $$e + {e^{ - 1}} = 2\left[ {1 + {1 \over {2!}} + {1 \over {4!}} + ......} \right]$$

$$\therefore$$ $${1 \over {2!}} + {1 \over {4!}} + {1 \over {6!}} + .......$$

$$ = {{e + {e^{ - 1}}} \over 2} - 1$$

$$ = {{{e^2} + 1 - 2e} \over {2e}}$$

$$ = {{{{\left( {e - 1} \right)}^2}} \over {2e}}$$

Solutions · 1