gimana ini tolong jawabin ..

jawab

u3  = 36
u7  = 24
(7- 3)b = 24 - 36
4b = - 12
b  = - 3

u3 = 36
a + 2b = a + 2(-3) = 36
a  - 6 = 36
a = 42

Sn = n./2 (2a + (n-1)b)
s30 = 15(84 + 29(-3))
s30 = 15 (-3)
s30= - 45