Question

obtenha a soma dos 51 primeiros termos da pa (6,9,12,15,18...)

Answer 1

a1 = 6     n = 51   r = 9 - 6 = 3  

S51 = ( a1 + an ) . n/2     S51 =[ ( 6 + an ) .3]/2

a51 = 6 + (51 - 1) . 3     a51 = 6 + 50 . 3   ⇒   a51 = 156 = an

S51 = ( 6 + 156) . 51 / 2    s51 =  162 . 51/2    s51 = 8262/2  S51 = 4131

Answer 2

Sn=(A1+An).n 2 S51=(6+51).51 2 S51=57.51 2 S51=2.907 2 S51=1.453,5