What is the angle between the planes `x + y + 2z = 6 and 2x - y - z =9` ?

Find the angle between the pari of lines given by (x -1)/(2) = (y -2)/(3) = (z -3)/(4) and (x -3)/(4) = (y -2)/(3) = (z -1)/(2).

Answer the question Find the value of lamda for which the planes 2x - 4y + 3z = 7 and x + 2y + lamda z = 18 are perpendicular to each other.