Diketahui suatu deret aritmatika u5=15 dan u14=51.hitunglah jumlah suku pertama

B= 51-15/14-5 =36/9=4

U5= u1+(n-1)b
15=u1+(4)4
15=u1+16
U1=15-16
U1=-1

Semoga bermanfaat

U5 = 15
U5 = a + 4b
15 = a + 4b (persamaan 1)

U14 = 51
U14 = a + 13b
51 = a + 13b (persamaan 2)

a +. 4b = 15
a + 13b = 51
------------------- -
-9b = -36
b = -36/-9
b = 4

a + 4b = 15
a + 4(4) = 15
a + 16 = 15
a = 15 - 16
a = -1

n = 1

Sn = n/2 × (2a + (n - 1) × b)
S1 = 1/2 × (2(-1) + (1 - 1) × 4)
S1 = 1/2 × (-2 + 0(4))
S1 = 1/2 × (-2 + 0)
S1 = 1/2 × (-2)
S1 = -1