pth, qth and rth terms of an arithmetic progression are a,b,c respectively then prove that a(q−r)+b(r−p)+c(p−q)=0
Let the first term and common difference of the A.P be A and D respectively
then
a=A+(p−1)D
b=A+(q−1)D
c=A+(r−1)D
Now,
a(q−r)+b(r−b)+c(p−q)
=[A+(p−1)D](q−r)+[A+(q−1)D(r−p)]+[A+(r−1)D](p−q)
=Aq−Ar+pdq−psr−Dq+Dr+Ar−Ap+qdr−qdp−dr+dp