Question

calcule a soma dos 15 primeiros termos da PA (100,94,88,82,...)

Answer 1

 a_n=a_1+(n-1).r

    a_{25}=100+(25-1).(-6)                 r=a_2-a_1=94-100=-6

    a_{25}=100+24.(-6)

    a_{25}=100-144

    a_{25}=-44

 

    S_n= \frac{(a_1+a_n).n}{2}  

    S_{25}= \frac{[100+(-44)].25}{2}  

    S_{25}= \frac{[100-44].25}{2}  

    S_{25}= \frac{56.25}{2}  

    S_{25}=28.25

    S_{25}=700

Answer 2

resolução!

r = a2 - a1

r = 94 - 100

r = - 6

a15 = a1 + 14r

a15 = 100 + 14 * (-6)

a15 = 100 + (-84)

a15 = 16

Sn = ( a1 + an ) n / 2

Sn = ( 100 + 16 ) 15 / 2

Sn = 116 * 15 / 2

Sn = 58 * 15

Sn = 870