Question
The fourth and ninth term of an a.p are -3 and 12 repectively find:the common different and the sum of the first seven term.
Answers
Reiny
a+3d = -3
a+8d = 12
subtract:
5d = 15
d = 15/5 = 3
a + 9 = -3
a = -12
common difference is 3
sum(7) = (7/2)(-24 + 6(3)) = -21
a+8d = 12
subtract:
5d = 15
d = 15/5 = 3
a + 9 = -3
a = -12
common difference is 3
sum(7) = (7/2)(-24 + 6(3)) = -21