3^n + 3^{n+1} = 3^n + 3×3^n
When you have for example: 3²×3³ then you have:
(3×3)×(3×3×3) = 3×3×3×3×3 = 3⁵ which is 3²⁺³
It works with any exponents, so:
if 3²⁺³ = 3²×3³ then 3ⁿ⁺¹ = 3ⁿ×3¹ = 3ⁿ×3 = 3×3ⁿ
and so:
3ⁿ + 3ⁿ⁺¹ = 3ⁿ + 3×3ⁿ
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