l|·sin26 4 而|a·ba|·|b·c0sθ|a×b|=a·|b·sin6 a·b||a×b||a21 b cos sin 2|b 2|b2 a. sin 20 →S d·b|·|a×b ABADD 2|b 因 a cos 20 d62| | sin 2 4 1 2 = a   而 | a  b |=| a |  | b | cos     | a b |=| a |  | b | sin     2 2 | | | | | | b a b a b          2 2 2 2 | | | | | | cos sin b a b      = | | sin 2 4 1 2 = a   2 2 | | | | | | b a b a b S BAD           = ② 因   | | cos 2 2 1 2 a d ds  =