In an A.P., `S_(3)=6` and `S_(6)=3` prove that:
`2(2n+1)S_(n+4)=(n+4)S_(2n+1)`.

If in an A.P. S_1=6 and S_7=105 , prove that : S_(n),S_(n-3)::(n+3),(n-3) .

Let the sum of n, 2n, 3n terms of an A.P. be S_1 , S_2 and S_3 , respectively, show that S_3 = 3(S_2 - S_1)

Show that S_(n)=(n(2n^(2)+9n+13))/(24) .

Let S_n denote the sum of first n terms of an AP and 3S_n=S_(2n) What is S_(3n):S_n equal to? What is S_(3n):S_(2n) equal to?

If a ,a_1, a_2, a_3, a_(2n),b are in A.P. and a ,g_1,g_2,g_3, ,g_(2n),b . are in G.P. and h s the H.M. of aa n db , then prove that (a_1+a_(2n))/(g_1g_(2n))+(a_2+a_(2n-1))/(g_1g_(2n-1))++(a_n+a_(n+1))/(g_ng_(n+1))=(2n)/h

If the coefficient of the rth, (r+1)th and (r+2)th terms in the expansion of (1+x)^(n) are in A.P., prove that n^(2) - n(4r +1) + 4r^(2) - 2=0 .