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Q.The position vectors of vertices of △ABC\triangle ABC are A(2i^−j^+k^)A(2\hat{i} - \hat{j} + \hat{k}), B(i^−3j^−5k^)B(\hat{i} - 3\hat{j} - 5\hat{k}) and C(3i^−4j^−4k^)C(3\hat{i} - 4\hat{j} - 4\hat{k}). Find all the angles of △ABC\triangle ABC.

CBSECBSE Class XII Board 2024Subjective· 3mImportance★★★★★
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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 cos⁡θ=a⃗⋅b⃗∣a⃗∣∣b⃗∣\cos \theta = \frac{\vec{a} \cdot \vec{b}}{|\vec{a}| |\vec{b}|} to find the angle between the side vectors originating from each vertex. The angles of △ABC\triangle ABC are arccos⁡(3541)\arccos\left(\frac{\sqrt{35}}{\sqrt{41}}\right), arccos⁡(641)\arccos\left(\frac{\sqrt{6}}{\sqrt{41}}\right), and 90∘90^\circ.

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 a⃗\vec{a} and b⃗\vec{b}, their dot product is defined as:

a⃗⋅b⃗=∣a⃗∣∣b⃗∣cos⁡θ\vec{a} \cdot \vec{b} = |\vec{a}| |\vec{b}| \cos \theta

where θ\theta is the angle between the two vectors. Rearranging this formula, we can find the cosine of the angle:

cos⁡θ=a⃗⋅b⃗∣a⃗∣∣b⃗∣\cos \theta = \frac{\vec{a} \cdot \vec{b}}{|\vec{a}| |\vec{b}|}

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 AB⃗\vec{AB} and AC⃗\vec{AC}. If we were to use AB⃗\vec{AB} and CA⃗\vec{CA}, we would be finding the angle between AB⃗\vec{AB} and the vector pointing from C to A, which is not the internal angle A of the triangle.

Watch out

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., AB⃗\vec{AB} and AC⃗\vec{AC} for angle A) or both terminate at the vertex (e.g., BA⃗\vec{BA} and CA⃗\vec{CA} for angle A). Using one originating and one terminating vector (e.g., AB⃗\vec{AB} and CA⃗\vec{CA}) will yield the exterior angle or its supplement, not the internal angle.

Let's apply this method step-by-step.

  1. Determine the side vectors of the triangle.

    Given the position vectors of the vertices:

    A⃗=2i^−j^+k^\vec{A} = 2\hat{i} - \hat{j} + \hat{k}

    B⃗=i^−3j^−5k^\vec{B} = \hat{i} - 3\hat{j} - 5\hat{k}

    C⃗=3i^−4j^−4k^\vec{C} = 3\hat{i} - 4\hat{j} - 4\hat{k}

    We find the vectors representing the sides. For angle A, we need AB⃗\vec{AB} and AC⃗\vec{AC}. For angle B, we need BA⃗\vec{BA} and BC⃗\vec{BC}. For angle C, we need CA⃗\vec{CA} and CB⃗\vec{CB}.

    • AB⃗=B⃗−A⃗=(i^−3j^−5k^)−(2i^−j^+k^)=(1−2)i^+(−3−(−1))j^+(−5−1)k^=−i^−2j^−6k^\vec{AB} = \vec{B} - \vec{A} = (\hat{i} - 3\hat{j} - 5\hat{k}) - (2\hat{i} - \hat{j} + \hat{k}) = (1-2)\hat{i} + (-3-(-1))\hat{j} + (-5-1)\hat{k} = -\hat{i} - 2\hat{j} - 6\hat{k}
    • AC⃗=C⃗−A⃗=(3i^−4j^−4k^)−(2i^−j^+k^)=(3−2)i^+(−4−(−1))j^+(−4−1)k^=i^−3j^−5k^\vec{AC} = \vec{C} - \vec{A} = (3\hat{i} - 4\hat{j} - 4\hat{k}) - (2\hat{i} - \hat{j} + \hat{k}) = (3-2)\hat{i} + (-4-(-1))\hat{j} + (-4-1)\hat{k} = \hat{i} - 3\hat{j} - 5\hat{k}
    • BC⃗=C⃗−B⃗=(3i^−4j^−4k^)−(i^−3j^−5k^)=(3−1)i^+(−4−(−3))j^+(−4−(−5))k^=2i^−j^+k^\vec{BC} = \vec{C} - \vec{B} = (3\hat{i} - 4\hat{j} - 4\hat{k}) - (\hat{i} - 3\hat{j} - 5\hat{k}) = (3-1)\hat{i} + (-4-(-3))\hat{j} + (-4-(-5))\hat{k} = 2\hat{i} - \hat{j} + \hat{k}

    We also need the reverse vectors for other angles:

    • BA⃗=−AB⃗=i^+2j^+6k^\vec{BA} = -\vec{AB} = \hat{i} + 2\hat{j} + 6\hat{k}
    • CA⃗=−AC⃗=−i^+3j^+5k^\vec{CA} = -\vec{AC} = -\hat{i} + 3\hat{j} + 5\hat{k}
    • CB⃗=−BC⃗=−2i^+j^−k^\vec{CB} = -\vec{BC} = -2\hat{i} + \hat{j} - \hat{k}
  2. Calculate the magnitudes of the side vectors.

    The magnitude of a vector xi^+yj^+zk^x\hat{i} + y\hat{j} + z\hat{k} is x2+y2+z2\sqrt{x^2 + y^2 + z^2}.

    • ∣AB⃗∣=(−1)2+(−2)2+(−6)2=1+4+36=41|\vec{AB}| = \sqrt{(-1)^2 + (-2)^2 + (-6)^2} = \sqrt{1 + 4 + 36} = \sqrt{41}
    • ∣AC⃗∣=(1)2+(−3)2+(−5)2=1+9+25=35|\vec{AC}| = \sqrt{(1)^2 + (-3)^2 + (-5)^2} = \sqrt{1 + 9 + 25} = \sqrt{35}
    • ∣BC⃗∣=(2)2+(−1)2+(1)2=4+1+1=6|\vec{BC}| = \sqrt{(2)^2 + (-1)^2 + (1)^2} = \sqrt{4 + 1 + 1} = \sqrt{6}

    Note that ∣BA⃗∣=∣AB⃗∣=41|\vec{BA}| = |\vec{AB}| = \sqrt{41}, ∣CA⃗∣=∣AC⃗∣=35|\vec{CA}| = |\vec{AC}| = \sqrt{35}, and ∣CB⃗∣=∣BC⃗∣=6|\vec{CB}| = |\vec{BC}| = \sqrt{6}.

  3. Calculate angle A.

    We use vectors AB⃗\vec{AB} and AC⃗\vec{AC}. …

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