Question

Obter a soma dos termos da P.A(100,95,…20)​

Answer 1

Resposta:

an=a1+(n-1).r

20=100+(n-1).-5

20=100-5n+5

5n=100+5-20

n=85/5

n=17

$s = \frac{(a1 + an) \times n}{2} \\ s = \frac{(100 + 20) \times 17}{2} \\ s = \frac{120 \times 17}{2} \\ s = 1020$

Answer 2

resolução!

r = a2 - a1

r = 95 - 100

r = - 5

an = a1 + ( n - 1 ) 5

20 = 100 + ( n - 1 ) - 5

20 = 100 + ( - 5n ) + 5

20 = 105 + ( - 5n )

20 - 105 = - 5n

- 85 = - 5n

n = 85 / 5

n = 17

Sn = ( a1 + an ) n / 2

Sn = ( 100 + 20 ) 17 / 2

Sn = 120 * 17 / 2

Sn = 60 * 17

Sn = 1020