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a=2·(1+3+3^2+....+3^20)

Răspuns :

a = 2×(1+3+3^2+....+3^20)
a = 2× (3^21 -1) : 2
a = 3^21 - 1

a=1+3+3^2+...+3^20
a=3^0+3^1+3^2+...+3^20 | ×3
3×a = 3^1 + 3^2 +3^3 +...+3^21
3×a-a = 3^21- 3^0
2×a = 3^21 - 1
a = (3^21 - 1) : 2