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1,2 4,5 7,8 49,50
3 , 9 , 15,....99
a= 3
b = 6
un = a + (n-1)b
99 = 3 + (n -1)6
99 = 3 + 6n - 6
99-3+6 = 6n
102 = 6n
n =17
S17 = 1/2n ( a+ u17)
= 1/2*17 ( 3 + 99)
= 8,5 ( 102)
= 867... C