Question

how many term of the ap 9 17 25 must be taken to give sum of 639

Answer 1

given ap=9,17,25......

here,

a=9 and d=17-9=8

here ,

Sn=n/2[a+(n-1)d]

639=n/2[9+(n-1)8]

639=n/2[9+8n-8]

639=n/2[1+8n]

639×2=n