Using your data in the definitions of AP ...
a+7d + a+8d = 72
2a + 15d = 72
a + 3d = -6
a = -6 - 15d
sub that into the first equation:
2(-6-15d) + 15d = 72
-12 -30d + 15d = 72
-15d = 84
d = -84/15
If the sum of 8th and 9th of an AP is 72 and 4th term is -6,find the common difference
2 answers
Thanks i want further explanation