{\(a_n\)} is an arithmetic sequence, \(a_5=20-a_{16}\) find the sum of the first 20 terms.
a5 + a16 = 20
a1 + 4d + a1 + 15d = 20
2a1 + 19d = 20
(a1) + ( a1 + 19d) = 20
(a1) + (a20) = 20
S20 = [ a20 +a1 ] [20 ] / 2 = [20] [10] = 200
+208 
