If 1+2+3+...+n=n (n+1), then 3 (1+3+5+....+89)=?  Choose the correct choice from the following
A) 6150
B) 6200
C) 6050
D) 6075
E) 5075
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; 89=1+(n-1)2   =&gt; n=45   Use value of n in (1)   then,   Sn = (45/2)(1+89) = 2025   Ans:- 3(Sn) = 6075   Answer : 6075
<result>D</result>
If 1+2+3+...+n=n (n+1), then 3 (1+3+5+....+89)=?