tentukan jumlah dari deret aritmatuka berikut ini ? A.47+49+51+.....+101 B.72+70+68+.....+40

A. 
a = 47
b = U2 - U1 = 2

Un = a + (n-1) b
101 = 47 + (n-1) 2
101 = 47 + 2n - 2
101 - 45 = 2n
56 / 2 = n
28 = n

Sn  = n/2 (a + Un)
S28 = 28/2 (47 + 101)
S28 = 14 (148)
S28 = 2072

B.
a = 72
b = U2 - U1 = -2

Un = a + (n-1) b
40 = 72 + (n-1) -2
40 = 72 - 2n + 2
40 - 74 = -2n
-34 / -2 = n
17 = n

Sn = n/2 (a + Un)
S17 = 17/2 (72 + 40)
S17 = 17/2 (112)
S17 = 1989 / 2
S17 = 994.5

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