Question

Cum aflu pe a?
a= 1+3+3²+........+3 la a 10 a+3 la a 11 a

Answer 1

S=1+3+3^2+...+3^11

3*S=3+3^2+3^3+...+3^12

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Scadem din a doua relatie pe prima=>

3S-S=3+3^2+3^3+...+3^12-1-3-3^2-...-3^11

=> 2S=3^12-1

=>S=$\frac{3^{12} -1}{2}$