<PRE><B>Question 843423</B>
S = ½(2t1 + (n-1)d)n

25 = ½(2t1 + 4d)5,
49 = ½(2t1+ 6d)7
t1 = 1,   d = 2

S12 = ½(2 + (11)2)12=144
tn = t1 + (n - 1)d
t12= 1 + (11)2=23

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