Question

Suku ke - n suatu deret aritmetika un: 3n-5. Rumus jumlah n suku pertama deret tersebut adalah

Answer 1

Un = 3n-5
a=U1 = 3.1-5 = -2
U2 = 3.2-5 = 1
b = U2-U1 = 1-(-2) = 3
Sn = n/2 (2a+(n-1)b)
     = n/2 (2.(-2)+(n-1)3
     = n/2 (-4+3n-3)
     = n/2 (3n-7)
     = (3n²-7n) / 2

Answer 2

Un = 3n-5
a= = 3.1-5 = -2
= 3.2-5 = 1
b = U2-U1= 1-(-2) = 3
Sn = n/2 (2a+(n-1)b)
     = n/2 (2.(-2)+(n-1)3
     = n/2 (-4+3n-3)
     = n/2 (3n-7)
     = (3n²-7n) / 2