Question

calcule a soma dos termos da PA (-27 , -18, ...,216​

Answer 1

Razão → a2-a1 → -18-(-27) → -18+27 = 9

An=a1+(n-1)·r

216=-27+(n-1)·9

216=-27+9n-9

216=-36+9n

216+36=9n

252=9n

n=252/9

n=28

---------------

Sn=(a1+an)·n/2

S28=(-24+216)·28/2

S28=192·14

S28=2688 → resposta

Answer 2

resolução!

r = a2 - a1

r = - 18 - (-27)

r = 9

an = a1 + ( n - 1 ) r

216 = - 27 + ( n - 1 ) 9

216 = - 27 + 9n - 9

216 = - 36 + 9n

216 + 36 = 9n

252 = 9n

n = 252 / 9

n = 28

Sn = ( a1 + an ) n / 2

Sn = ( - 27 + 216 ) 28 / 2

Sn = 189 * 14

Sn = 2646