Question

Determine a soma de vinte termos da PA (1,8,15... )

Answer 1

Resolução!


r = a2 - a1

r = 8 - 1

r = 7



an = a1 + ( n - 1 ) r

an = 1 + ( 20 - 1 ) 7

an = 1 + 19 * 7

an = 1 + 133

an = 134


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Sn = ( a1 + an ) n / 2

Sn = ( 1 + 134 ) 20 / 2

Sn = 135 * 20 / 2

Sn = 2700 / 2

Sn = 1350