Question

If 1+4+7+10+ +x equal 287

Answer 1

Answer:

a=1, d=7-4=3,Sn=287,n=?

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

287 =n/2 [2×1+(n-1)3]

287 =n/2[2+3n-3]

287 = n/2(-1+3n)

574 =-n+3n^2

3n^2-n-574=0

3n^2-42n+41n-574=0

3n(n-14)+41(n-14)=0

(n-14)(3n+41)=0

n-14=0,3n+41=0

n=14,n=-41/3(which is not possible)

so n=14