Square of Sum of Two Vectors

Rarely Tested

Square of Sum of Two Vectors

$$|\vec a+\vec b|^2=|\vec a|^2+|\vec b|^2+2\vec a\cdot\vec b$$

Usage

- Used to calculate the magnitude of a vector sum without explicitly finding the angle.

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Question 1

Let $$\vec{a},\vec{b}$$ be two vectors, and let $$P,Q$$ and $$R$$ be the points with position vectors $$\vec{a}$$, $$\vec{b}$$ and $$\vec{a}+\vec{b}$$, respectively, with respect to the origin $$O$$. If $$|\vec{a}+\vec{b}|=\sqrt{21}$$, $$|\vec{a}-\vec{b}|=3$$, and $$\vec{a}$$ and $$(\vec{a}-\vec{b})$$ are perpendicular to each other, then the area of the triangle $$OPR$$ is

Question 2

If $$\vec a,\vec b,\vec c$$ are non-zero vectors such that $$|\vec a+\vec b+\vec c|^2=|\vec a|^2+|\vec b|^2+|\vec c|^2,$$ then the value of $$\vec a\cdot\vec b+\vec b\cdot\vec c+\vec c\cdot\vec a$$ is:

Question 3

Let $$\vec{a}$$ and $$\vec{b}$$ be two orthogonal vectors such that $$|\vec{a}| = 1$$ and $$|\vec{b}| = 2$$. If a variable vector $$\vec{x}$$ satisfies the vector equation:
$$\vec{x} + \vec{a} \times \vec{x} = \vec{b}$$
Find the value of the squared magnitude expression $$10|\vec{x}|^2$$.

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