Q.The position vectors of vertices of are , and . Find all the angles of .
You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.
Start your 14-day free trial to unlock the full solution →To find the angles of a triangle given its vertices, we first determine the vectors representing its sides. Then, we use the dot product formula to find the angle between the side vectors originating from each vertex. The angles of are , , and .
The core idea behind finding the angles of a triangle when its vertices are given as position vectors is to leverage the geometric interpretation of the dot product. A position vector simply tells us the coordinates of a point relative to the origin. To find the vector representing a side of the triangle, say from point A to point B, we subtract the position vector of A from the position vector of B.
Once we have the vectors representing the sides, we can use the dot product formula. For any two vectors and , their dot product is defined as:
where is the angle between the two vectors. Rearranging this formula, we can find the cosine of the angle:
To find an internal angle of a triangle, say angle A, we must consider two vectors that originate from (or terminate at) vertex A and form the sides of the triangle. For example, we would use vectors and . If we were to use and , we would be finding the angle between and the vector pointing from C to A, which is not the internal angle A of the triangle.
When calculating an internal angle of a triangle, ensure the two vectors you use for the dot product either both originate from the vertex (e.g., and for angle A) or both terminate at the vertex (e.g., and for angle A). Using one originating and one terminating vector (e.g., and ) will yield the exterior angle or its supplement, not the internal angle.
Let's apply this method step-by-step.
-
Determine the side vectors of the triangle.
Given the position vectors of the vertices:
We find the vectors representing the sides. For angle A, we need and . For angle B, we need and . For angle C, we need and .
We also need the reverse vectors for other angles:
-
Calculate the magnitudes of the side vectors.
The magnitude of a vector is .
Note that , , and .
-
Calculate angle A.
We use vectors and . …
Unlock everything free for 14 days
- Full step-by-step solutions
- Concept-first explanations
- Methods, shortcuts & mistakes
- PYQ mapping + timed mock tests
Full access for 14 days. No credit card required.