dari barisan aritmetika diketahui U3 =18 dan U7 = 38. jumlah 24 suku pertama adalah???

Dik. barisan deret aritmatika :
U3 = 18 & U7 = 38
Dit S24 ?

Jawab...

U3 = a + 2b = 18
U7 = a + 6b = 38
----------------------- -
@ -4b = -20
@ b = 5

a :
U3 = a + 2b = 18
@ a + 2(5) = 18
@ a + 10 = 18.
@ a = 8

Sn = n/2 (2a+(n-1)b)
S24 = 24/2 (2(8) + (23) (5))
S24 = 12 (16 + 115)
S24 = 12 (131)
S24 = 1572


Semoga membantu.. ^_^