|a|=3, |b|=4, |c|=5, a+b+c=0a2+b2=c2 故向量 a、b、c 构成一个直角三角形,斜边为 c,直角边为 a 和 b| a x b + b x c + c x a | =| a x b + b x (-a-b)+(-a-b) x a |=| a x b - b x a - b x b - a x a - b x a |=| a x b + a x b + a x b |=3 | a x b |=3 |a| |b| sinC=3 x 3 x 4 x sin90°=36...