Question

which term of the Ap 3,15,27,... Wimbe 120 More than its 21 tearm

Answer 1

Given A.P is 3,15,27......
Here a(first term)=3
         d (common difference) = 12
Let the term of A.P which is 120 more than its 21st term = a+(n-1)d
Therefore a+(n-1)d = 21st term +120
                 3 + (n-1)12 = a+20d + 120
                 3+12n-12 = 3+20*12+120
                  12n - 9 = 123+240
                      12n = 363+9
                        n = 372/12
⇒ n=31 st term
Therefore the 31st term will be 120 more than its 21st term