diketahui deret hitung suku ke 5:25 suku ke 9:29 berapa suku ke 15 berapa jumlahnya

U5 = a + 4b = 25
U9 = a + 8b = 29
---------------------------(-)
- 4b = - 4
b = 1
a + 4b = 25
a + 4(1) = 25
a = 25 - 4
a = 21
U15 = a + 14b = 21 + 14(1) = 21 + 14 = 35
S15 = ½ . 15 (21 + 35)
= 15/2 . 56
= 15 × 28
= 420