Question

a soma dos 20 primeiros termos da PA (7,4...

Answer 1

PA
a1 = 7
r = 4 - 7 ⇒ r = - 3
n = 20
an = a20 = ?

an = a1 + ( n - 1 ) . r
a20 = 7 + ( 20 - 1) . ( - 3)
a20 = 7 + ( 19 ) . ( - 3)
a20 = 7 - 57
a20 = - 50

S20 = (a1 + an). n 
                2
S20 = (7 - 50 ) . 20
                2
S20 = - 43 . 20 
               2
S20 = - 860
              2
S20 = - 430

A soma dos 20 primeiros termos é - 430. 

PA = ( 7, 4, 1, - 2, - 5, - 8, - 11, -14, - 17, - 20, - 23, - 26, - 29, - 32, - 35, - 38, - 41, - 44, - 47, - 50 )

Answer 2

a1=7
r=-3
an=a1+(n-1)r
a20=7+19.(-3)
a20=7-57
a20=-50

Sn=(a1+an).n/2
s20=(7+(-50)).20/2
s20=(-43).20/2
s20=-860/2
s20=-430

(7,4,1,-2,-5,-8,-11,-14,-17,-20,-23,-26,-29,-32,-35,-38,-41,-44,-47,-50)

A soma dos 20 primeiro termos é -430.