$ \sum _{n=1}^{\infty }\zeta \left( 2n\right) a^n $ simplify the given function as depending on $a$

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Q: $ \sum _{n=1}^{\infty }\zeta \left( 2n\right) a^n $: simplify the given function as depending on $a$.

I tried to solve it by using Dirichlet zeta function but it didn't work I will be so gratefull if you guys prove it.