方向向量S=(cosπ/3,cosπ/4,cos2π/3)=(1/2,√2/2,-1/2)得直线的对称式方程为:
(x-2)/(1/2)=(y-2)/(√2/2)=(z+1)/(-1/2)
(x-2)=(y-2)/√2=(z+1)/(-1)