If 1+2+3+...+n=n (n+1), then 3 (1+3+5+....+69)=? Choose one:
A) 3675 B) 3575 C) 3475 D) 3375 E) 3275

Rationale: explanation : to solve this use the formula of ap , sn = ( n / 2 ) ( a + l ) . . . . . . . . . . . . . . . . ( 1 ) to find n , use = > tn = a + ( n - 1 ) d = > 69 = 1 + ( n - 1 ) 2 = > n = 35 use value of n in ( 1 ) then , sn = ( 35 / 2 ) ( 1 + 69 ) = 1225 ans : - 3 ( sn ) = 3675 answer : a
Answer: A