The number of all three element subsets of set {a1, a2, a3, ...... An} which contain az is n C3 n+1C3 n-1C2

The number of all three element subsets of the {a_1,a_2 ,a_3 ,… a_n} which contain a_3 is

Find the number of all three elements subsets of the set {a_1, a_2, a_3, ........... a_n} which contain a_3dot

Find the number of all three elements subsets of the set {a_1, a_2, a_3, a_n} which contain a_3dot

Find the number of all three elements subsets of the set {a_1, a_2, a_3, a_n} which contain a_3dot

Find the number of all three elements subsets of the set {a_1, a_2, a_3, ........... a_n} which contain a_3dot

The number all three element ofsubsets of the set A = {a_1 ,a_2 , . . . A_8} contains the element a_3 is

If one quarter of all three element subsets of the set A = { a_1 ,a_2,a_3,. . . ,a_n} contains the element a_3 then n=

FOr an integer nlt2, we have real numbers a_1,a_2,a_3,a_4,..........,a_n such that a_2+a_3+a_4+.......+a_n=a1 and a_1+a_3+a_4+...........+a_n=a_2...............a_1+a_2+a_3+..............+a_(n-1)=a_n what is the value of a_1+a_2+a_3+........+a_n?

If a_(n) = n(n!) , then what is a_1 +a_2 +a_3 +......+ a_(10) equal to ?