Which term of the A.P. 21, 42, 63, 84, …… is 420 ?

Asked by Topperlearning User | 22nd Sep, 2017, 02:54: PM

Expert Answer:

Given A.P. is 21, 42, 63, 84, ….. 420

 begin mathsize 12px style straight a subscript 1 space equals space 21 comma space straight a subscript 2 space equals space 42 end style

 begin mathsize 12px style straight a subscript 3 space equals space 63 comma space straight a subscript 4 space equals space 84 comma.... end style

begin mathsize 12px style straight d space equals space straight a subscript 2 space minus space straight a subscript 1 space equals space 42 space minus space 21 space equals space 21 end style

begin mathsize 12px style Let space 420 space be space the space straight n to the power of th space term space using space straight a subscript straight n space equals space straight a subscript 1 space plus space open parentheses straight n space minus space 1 close parentheses straight d
21 space plus space open parentheses straight n space minus space 1 close parentheses cross times 21 space equals space 420
21 space plus space 21 straight n space minus space 21 space equals space 420
21 straight n space equals space 420
straight n space equals space 20
end style


                                     


           

Answered by  | 22nd Sep, 2017, 04:54: PM