Question

calcule a soma dos termos PA (60, 52, 44,...,-20)
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Answer 1

an = a1+(n—1)*r

Onde:

r = 52—60 = —8

Logo:

—20 = 60+(n—1)*—8

—20 = 60+(—8n)+8

—20—60—8 = —8n

—88 = —8n

n = 88/8

n = 11

Soma:

Sn = (a1+an)*n/2

S11 = (60+(—20))*11/2

S11 = 40*11/2

S11 = 440/2

S11 = 220

Dúvidas??Comente!!

Answer 2

resolução!

r = a2 - a1

r = 52 - 60

r = - 8

an = a1 + ( n - 1 ) r

- 20 = 60 + ( n - 1 ) - 8

- 20 = 60 + ( - 8n ) + 8

- 20 = 68 + ( - 8n )

- 20 - 68 = - 8n

- 88 = - 8n * (-1)

n = 88/8

n = 11

Sn = ( a1 + an ) n / 2

Sn = ( 60 + (-20 ) 11 / 2

Sn = 40 * 11 / 2

Sn = 20 * 11

Sn = 220