Jawabanmu

2014-02-18T21:38:52+07:00
7+14+21+....+161
b= 7

161 = 7 + (n-1) . 7
161 = 7 + 7n + 7
147 =  7n
n = 21

Sn = n/2 ( U1 + Un)
         21/2 ( 7 + 161 )
           21/2 (168)
           1764
2014-02-18T21:49:54+07:00
Un = a+(n-1)b
161 = 7+(n-1)7 
161 = 7n , n = 23

S23 = 1/2 x 23 x (7+161) = 1932

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