diketahui barisan aritmatika dengan U3=21 dan U7+U8=69 suku ke-10 dari barisan tersebut adalah...

Barisan dan deret

U3 = 21
a + (3 - 1) b = 21

a + 2b = 21

U7 + U8 = 69
a + 6b + a + 7b = 69
2a + 13b = 69


a + 2b = 21 ( × 2)
2a + 13b = 69 (× 1)

2a + 4b = 42
2a + 13b = 69
___________ _
-9b = -27
b = 3

a + 2b = 21
a + 2(3) = 21
a + 6 = 21
a = 21 - 6
a = 15

U10 = a + ( n - 1) b
U10 = 15 + (10 - 1) 3
U10 = 15 + 9 × 3
U10 = 15 + 27
U10 = 42