Question

how many types of ap : 9, 17, 25,.... must be taken to give a sum of 636?

Answer 1

Hey its terms not types.

Btw..

Answer 2

a= 9 , d= 8 , sn = 636
sn = n/2 { 2a +( n -1 ) d }
636 = n/2 { 2*9 + ( n-1) 8}
636 = n/2 { 18 + 8n - 8}
636= n/2 { 10 + 8n }
636 = n/2 * 2 { 5 + 4n}
636 = 5n + 4n^2
4n^2 + 5n - 636 = 0
now substitute by x = - b +- √ b^2 - 4ac/ 2a
then the will be n= 12