If 1+2+3+...+n=n (n+1), then 3 (1+3+5+....+79)=?	Options:
A) 4500
B) 4250
C) 4350
D) 4800
E) 4150
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; 79=1+(n-1)2   =&gt; n=40   Use value of n in (1)   then,   Sn = (40/2)(1+79) = 1600   Ans:- 3(Sn) = 4800   Answer : 4800
<result>D</result>
If 1+2+3+...+n=n (n+1), then 3 (1+3+5+....+79)=?