Calcula la suma de los primeros 28 terminos de esta sucesion 42, 49, 56, 63, 70,...
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primero hallemos la razon de la sucesion
42.....49.......56............63..........70
......+7......+7..........+7............+7
la razon es R= 7
el primer termino es T1=42
Recordemos que
Tn=T1+R(n-1)
Tn=42+7(n-1)=7n+35
Ahora aplicando sumatoria
S=Σ(7n+35){k=1 a 28}
Aplicando propiedades
S=Σ7n{k=1 a 28}+Σ35{k=1 a 28}
S=7Σn{k=1 a 28}+35n {n=28}
S=7n(n+1)/2 {n=28}+35(28)
S=7(28)(29)/2+980
S=3822 rpta