tentukan jumlah 25 suku pertama dari deret aritmatika 35 + 31 + 27 + 23

Suku awal (a) = 35
Beda (b) = U2 - U1 = 31 - 35 = -4
S25 = ?
Sn = n/2 ( 2a + (n-1) b )
S25 = 25/2 ( 2 × 35 + (25-1) -4)
S25 = 25/2 (70 + 24 × -4)
S25 = 25/2 (70 - 96)
S25 = 25/2 × -26
S25 = -325

S25 ....?

Dik : a = 35
b = -4

Sn = n/2 (2a + (n-1)b )
S25 = 25/2 (2(35) + (25-1)-4)
S25 = 25/2 (70 + (-96))
S25 = 25/2 (-26)
S25 = 25 x (-13)
S25 = -325

atau cara satunya

Un = a+(n-1)b
U25 = 35 + (24).-4
U25 = 35 + (-96)
U25 = -61

Sn = n/2 (a+Un)
S25 = 25/2 (35+(-61)
S25 = 25/2 (-26)
S25 = 25 x (-13)
S25 = -325