Page 235 - aljaber

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an = a1 + (n − 1)d

−20 = 50 + (n − 1) × − 1 
2

−70 = − 1 n + 1
22

−70 − 1 = − 1 n
22
− 141 = − n
22
n = 141

S141 = 141 (50 + (−20)) = 141 × 30 = 141× 15
2 2

S141 = 2115

()

.

171

. 1, 2, 3, 4,…