dalam barisan aritmatika diketahui U3= 13, U7= 33. Jumlah 21 suku pertama adalah ….

U3=13
U7=33

U3=a+(3-1)b
U3=a+2b = 13

U7=a+(7-1)b
U7=a+6b=33

a+2b=13
a+6b=33
_______ _
-4b=-20
b=5

a+2b=13
a+2(5)=13
a+10=13
a=13-10
a=3

S21=?

S21=n/2(2a+(n-1)b)
S21=21/2(2×3+ (21-1)5)
S21=10,5(6+20(5))
S21=10,5(6+100)
S21=10,5(106)
S21=1.113

Semoga Membantu +__+

Barisan Aritmatika
U3 = a + 2b = 13
U7 = 33

beda = (U7 - U3)/(7 - 3)
b = (33 - 13)/4
b = 5

U3 = 13
a + 2b = 13
a = 13 - 2b
a = 13 - 2.5
a = 13 - 10
a = 3

Jumlah 21 suku pertama = S21
Sn = n/2 (2a + (n - 1)b)
S21 = 21/2 (2.3 + 20.5)
S21 = 21/2 (6 + 100)
S21 = 21/2 × 106
S21 = 21 × 53
S21 = 1113