Question

Solve

1+4+7+10.…..….......+x =590

Answer 1

first you can see that it firms an AP

now a = 1 ,d = 4 - 1 = 3 , an = x

now an = a + (n-1) d

x = 1 + (n-1) (3)

n = x + 2/3

now putting the value of n

sn = x + 2/32 {2(1) + ( x + 2/3 - 1) (3) }

2x + 4/3 { 2 + x + 2 - 3/3 ( 3 ) }

2 x + 4/3 (2 + x - 1 )

2 (2x2 + 3x - 2 ) /3 solving this you will get answer x=58