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Step-by-step explanation:
the given series :
5+7+13+31+85...up to n terms
(1+4)+(3+4)+(9+4)+(27+4)+(81+4) ...up to n terms
(1+3+9+27+81+....up to n terms)+(4+4+..up to n terms)
we know the formula for g.p when r>0
a(rⁿ-1)/(r-1)
so,
1(3ⁿ-1)/(3-1)+4*n
=>(3ⁿ-1)/2+4n
=>1/2(3ⁿ+8n-1)
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