Bantuin caranya pliss Dari deret aritmatika diketahui U3 = 7, sedangkan U4+U7 = 29, jumlah 27 suku pertamanya adalah....

U3 = 7
a + 2b = 7

U4 + U7 = 29
(a + 3b) + (a + 6b) = 29
2a + 9b = 29

eliminasi:
2a + 4b = 14
2a + 9b = 29
__________ --
5b = -15
b = 3
a = 1

S27 = (27/2)(2(1) + 26(3))
= (27/2)(80)
= 1080

Langsung aja y

U3 = 7
a + 2b = 7

U4 + U7 = 29
a + 3b + a + 6b = 29
2a + 9b = 29

a + 2b = 7 (×2)
2a + 9b = 29 (×1)

2a + 4b = 14
2a + 9b = 29
__________-
-5b = -15
b = 3

a + 2b = 7
a = 7 - 2(3)
a = 7 - 6
a = 1

S27 = 27/2 (2a + 26b)
S27 = 13,5 (2.1 + 26.3)
S27 = 13,5 (2 + 78)
S27 = 13,5 (80)
S27 = 1.080

semoga berguna +_+