Question

cos(170° + a)-sin(100° -a) /sin(280º - a)​

Answer 1

Ответ:

Sn = 120, a1 = 10, n = 6.

Sn = ((2a1 + d(n – 1))n)/2,

120 = ((20 + 5d)6)/2,

d = 4,

an = a1 + d(n-1),

a3 = 10 + 4 . 2 = 18