If vectors P =a i +a j +3 k and Q=a i -2 j - k are perpendicular to each other, then the positive value of a is

Step-by-step solution

Vector P =a i +a j +3 k and vector Q =a i -2 j - k . If two vectors are perpendicular to each other, then P Q =0 or (a i +a j +3 k ) (a i -2 j - k )=0 or a^2-2 a-3=0 . Solving this quadratic equation, we get a=3 or -1 . Therefore positive value of a is 3 .