Dari barisan aritmetika diketahui suku ke-3 = 14 dan suku ke-7 = 26. Jumlah 18 suku pertama adalah... #dibantu ya kakak, plus cara. Terimakasih

jawab =
[tex] b \: = \frac{u7 - u3}{7 - 3 } = \frac{26 - 14}{4} = \frac{12}{4} = 3 \\ u18 = a + (n - 1)b \: \\ = 8 + (18 - 1)3 \\ = 8 + (17)3 \\ = 8 + 51 = 59[/tex]

U₃ = a + (3 - 1)b
U₇ = a + (7 - 1)b
14 = a + 2b
26 = a + 6b
__________ -
-12 = -4b
b = -12 / -4
b = 3. . . . . . . . . . . b
a + 2b = 14
a + 2(3) = 14
a + 6 = 14
a = 14 - 6
a = 8. . . . . . . . . . . a

S18 = 18/2 . (2(8) + (18 - 1) x 3)
= 9(16 + 17 x 3)
= 9(16 + 51)
= 9(67)
= 603