If 1+2+3+...+n=n (n+1), then
2 (1+3+5+....+99)=?  Pick:
A) 6000  B) 5000  C) 2500  D) 6500  E) None of these
Explanation :   To solve this use the formula of AP,   Sn= (n/2)(a+l) ................(1)   To find n, use   =&gt; Tn= a+(n-1)d   =&gt; 99=1+(n-1)2   =&gt; n=50   Use value of n in (1)   then,   Sn = (50/2)(1+99) = 2500   Ans:- 2(Sn) = 5000   Hence (5000) is correct   Answer : 5000
<result>B</result>
If 1+2+3+...+n=n (n+1), then
2 (1+3+5+....+99)=?