Question 1166717
A theatre has 20 rows of chairs . The first row has 16 chairs , the second has 18, the third has 20 , and so on . In total , the theatre seats how many people?
solution 
16,+18,+20.+...............+.20th 
this is problem of sum of AP
a=16 d=2   n=20  we have to find s20=?
 a20=a+19d 
l (last term)=16+19x2
=16+38=54
sn=n(a+l)/2=20(16+54)/2
s20=20x70/2=700
total 700  seats