posted by Nina
Given the third term of an arithmetic sequence less than the fourth term by three. The seventh term is two times the fifth term. Find the common difference and the first term.
since the terms have a constant difference, the difference is 3.
a+6*3 = 2(a+4*3)
a+18 = 2a+24
a = -6
sequence is -6,-3,0,3,6,9,12,15,18,21,24
T7 = 12 = 2*6 = 2T5
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