tentukan nilai u 15+u 20 dari barisan aritmatika berikut a.64,61,58,55 b.-27,-19,-11 pakai caraa

a ). a = 64
b = 61 - 64
= - 3

Un = a + ( n - 1 ) × b
U15 = 64 + 14 × ( -3 )
U15 = 64 + ( - 42 )
U15 = 22

U20 = 64 + 19 × ( - 3 )
U20 = 64 + ( - 57 )
U20 = 7

U15 + U20
= 22 + 7
= 29

b ). a = - 27
b = - 19 - ( - 27 )
= 8

Un = a + ( n - 1 ) × b
U15 = ( - 27 ) + 14 × 8
U15 = ( - 27 ) + 112
U15 = 85

U20 = ( - 27 ) + 19 × 8
U20 = ( - 27 ) + 152
U20 = 125


U15 + U20
= 85 + 125
= 210