Find the angle between the vectors with direction ratios proportional to 1,–2,1 and 4,3,2.
Let us assume the direction ratios of vectors be (r1,r2,r3) and (r4,r5,r6).
Then,
⇒ (r1,r2,r3) = (1,–2,1)
⇒ (r4,r5,r6) = (4,3,2)
We know that the angle between the vectors with direction ratios proportional to (a1,b1,c1) and (a2,b2,c2) is given by:
⇒
Using the above formula we calculate the angle between the vectors.
Let be the angle between the two vectors given in the problem.
⇒
⇒
⇒
⇒
⇒
∴ The angle between two given vectors is .