Question

calcule a soma dos termos da pg finita(5,20,...,1280)

Answer 1

a1 = 5
a2 = 20
q = 20/5 = 4 ***
an = 1280

1280 = 5 * 4ⁿ⁻¹
4ⁿ⁻¹ = 1280/5 = 256
(2²)ⁿ⁻¹  = 2⁸
2²ⁿ⁻²    = 2⁸
2n - 2 = 8
2n = 8 + 2 = 10
2n = 10
n = 5 *****

S5  = a1 ( qⁿ   - 1 )/ ( q - 1)
S5 = 5 ( 4⁵   - 1 ) / ( 4 - 1 )
S5 = 5 ( 1024  - 1 )/3
S5 = 5 ( 1023) / 3
S5 =5115/3 = 1705 ***