Question

in a series 21,18,15,..........,which term is -81?

Answer 1

On This Joyous Day, And throughout the coming year, May your be filled with good luck and prosperity.�

Answer 2

Let the nth term of the above given series be -81

Series = 21, 18, 15, ...........
From the given Arithmetic Progression, 

First term, a= 21
Difference, d = 18-21 = -3
 Now to calculate the nth Term of the above AP we have the Formula as,
 
                          a(n) = a + (n-1) * d
 Thus putting the values from above
               
                       a (n) = 21 + (n-1) (-3)
                               = 21 - 3n +3
                               = 24 -3n
 Now from the question we can assume a(n) to be -81
  Thus,                    24 - 3n = -81
                                      3n = 24 + 81
                                      3n = 105
                                        n = 35
        Therefore 35th term of the above series is -81