Q.Find the magnitude of the resultant of two vectors $\vec{A}$ and $\vec{B}$ and angle $\theta$ between them.

Triangle Inequality (Bounding the Resultant of Two Vectors)

When you combine two displacements, two velocities, or two forces in "Motion in a Plane," you add them as vectors using the triangle law: place the tail of the second vector at the head of the first, and the resultant runs from the start to the finish. The triangle inequality is simply the statement that this resultant can never be longer than the two vectors laid end to end, and can never be shorter than their difference.

The Intuition

Suppose you walk 3 m in one direction, then 4 m in some other direction. Could you end up 8 m from where you started? No — the farthest you can possibly get is $3+4=7$ m, and that only happens if both walks point the same way, so there's no bend at all (a "flat" triangle). The moment the second walk points in a different direction, a real corner appears, and cutting across that corner (the direct path) is always shorter than going via the corner. That's the geometric heart of every triangle: any one side is shorter than the sum of the other two, unless the triangle collapses onto a straight line.

The Precise Statement

For two vectors $\vec{A}$ and $\vec{B}$ added by the triangle law, the magnitude of the resultant $\vec{R} = \vec{A} + \vec{B}$ is bounded on both sides:

• The upper bound $|\vec{A}+\vec{B}| \le |\vec{A}|+|\vec{B}|$ is reached only when $\vec{A}$ and $\vec{B}$ point in exactly the same direction (the angle between them is $0^\circ$) — the triangle flattens out.
• The lower bound $|\vec{A}+\vec{B}| \ge \big||\vec{A}|-|\vec{B}|\big|$ is reached only when $\vec{A}$ and $\vec{B}$ point in exactly opposite directions (the angle is $180^\circ$).
• For any angle in between, the resultant magnitude lies strictly between these two limits.

This is the vector form of the ordinary triangle inequality $|x+y| \le |x|+|y|$ you may already know for numbers: here $x$ and $y$ become vectors, and "the sides of a triangle" become "a vector, another vector, and their sum."

Why It Matters in Kinematics

This bound is genuinely useful when combining physical quantities in the plane:

• Relative velocity: if a boat has speed 5 m/s relative to water and the river flows at 3 m/s, the boat's speed relative to the ground must lie between $|5-3|=2$ m/s and $5+3=8$ m/s, depending on the angle the boat is steered — it can never be less than 2 or more than 8.
• Combining forces or displacements: if you know only the magnitudes of two vectors, not the angle between them, this inequality instantly tells you the range of possible resultant magnitudes without doing any trigonometry. …