Insert six A.M.s between 15 and -13.
Let A1,A2,A3,A4,A5,A6 be the 6 A.M's between 15 and -13
Then,
15,A1,A2,A3,A4,A5,A6,−13 are in A.P. of 8 terms
Here, −13=a8=a+7d
⇒−13=15+7d
Or d = -4 ..... (i)
∴A1=a+d=15−4=11
A2=a+2d=15−2(4)=7
A3=a+3d=15−4(3)=3
A4=a+4d=15−4(4)=−1
A5=a+5d=15−4(5)=−5
A6=a+6d=15−4(6)=−9
The 6 A.M.s between 15 and -13 are 11, 7, 3, -1, -5 and -9.