Problema Solution

Find 5 consecutive terms of an A.P. whose sum is 45 and the sum of cube of 2nd and 4th term is 1944

Answer provided by our tutors

Let the terms of the A.P. be:

a1 = a

a2 = a + d

a3 = a + 2d

a4 = a + 3d

a5 = a + 4d

The sum of the 5 consecutive terms is:

S5 = (5/2)(a1 + a5)

and also S5 = 45:

45 = (5/2)(a + a + 4d)

(5/2)(2a + 4d) = 45

2a + 4d = 18 divide both sides by 2

a + 2d = 9

a = 9 - 2d

The sum of cube of 2nd and 4th term is 1944:

a2^3 + a4^3 = 1944

(a + d)^3 + (a + 3d)^3 = 1944

plug a = 9 - 2d into the above equation:

(9 - 2d + d)^3 + (9 - 2d + 3d)^3 = 1944

by solving we find :

d1 = 3

d2 = -3

For d1 = 3 we have a = 9 - 2*3 = 3 thus the terms are:

3, 6, 9, 12 and 15

For d2 = -3 we have a = 9 - 2*(-3) = 15 thus the terms are:

15, 12, 9, 6  and 3.