Question

Suatu deret aritmatika, suku ke-7 = 14 dan suku ke-11 = 26, jumlah 10 suku pertama deret tersebut adalh?
Tolong penyelesaian juga yaa

Answer 1

-aritmatika-

u7 = 14
u11 = 26

un = a+(n-1)b
u7 = a+(6)b = 14
u11 = a+(10)b =26
----------------------- -
-4b = -12
b = 3
u7 = a+6(3) = 14
a = 14-18
a = -4

sn = n/2 {2a+(n-1)b}
s10 = 10/2 {(-8)+(9)3}
s10 = 5 {(-8)+27}
s10 = 5 (19)
s10 = 95

Answer 2

Suku ke 7 = a+6b = 14---------(1)
suku ke 11 = a+10b =26-------(2)
(2) -(1) 4b=12, b=3 
a=-4
Jumlah suku ke 10 =n/2 (2a+(n-1)b) = 5 (-8 + 27) = 95