Expression 3: polygon left parenthesis, cos left parenthesis, 21 "a" , right parenthesis plus sin left parenthesis, 7 "a" , right parenthesis , sin left parenthesis, 14 "a" , right parenthesis plus sin left parenthesis, 21 "a" , right parenthesis , right parenthesispolygoncos21a+sin7a,sin14a+sin21a