Question

find the sum of the first 19 terms of the ap, whose nyh term is given by tn=7-4n

Answer 1

$s = 7 - 4n \\ s |1| = a |1| = 3 \\ s |2| = - 1 \\ s |2 | = a |1| + a |2| \\ - 1 = 3 + a |2| \\ a |2 | = - 4 \\ d = a |2| - a |1| = - 4 - ( 3) = - 7\\ \\ s |19| = (19 \div 2)(2(3) + 18( - 7)) \\ s = (19 \div 2)(6 - 126) \\ s = - 19(110) = - 2090$
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