Question

Diketahui barisan aritmatika u3=15 dan u7=31
tentukan
a. beda
b.suku pertama
c.jumlah 80 suku pertama

Answer 1

A) u3 = 15
u7 = 31

a + 2b = 15
a + 6b = 31
--------------------(-)
-4b = - 16
b = 4
b) a + 2b = 15
a + 2(4) = 15
a + 8 = 15
a = 7
c) S80 = 80/2(2.7 + (80 - 1)4)
= 40(14 + (79)4)
= 40(14 +316)
= 40(330)
= 13200

Answer 2

U3 => a+2b=15
U7 => a+6b=31
------------- -
-4b=-16
b=4 (beda)
U3 => a + 2b =15
a +2(4)=15
a = 7 (suku pertama)
U80 => a + 79b = 7 + 79(4)
= 323