# Math Help - Precal, proof for distance between 2 polar points

1. ## Precal, proof for distance between 2 polar points

Use the Law of Cosines to prove that the distance between 2 polar points (r1, θ1) and (r2, θ2) is d2=r12+r22-2r1r2cos(θ2-θ1)

2. Hello, rwc123!

I don't see any difficulty . . . am I missing something?

Using the Law of Cosines

. . prove that the distance $d$ between 2 polar points $A(r_1,\:\theta_1)\text{ and }B(r_2,\:\theta_2)$

. . is given by:. . $d^2\;=\;r_1^2+ r_2^2 -2(r_1)(r_2)\cos(\theta_2-\theta_1)$
Code:
                            B
*
*   *
*       * d
*           *
*               *
r2   *                   * A
*                *
*  θ2 - θ1    *
*        *    r1
*     *
*  *
O *

We have: . $\begin{array}{ccccccc}
|\overrightarrow{OA}| &=& r_1 \\ \\[-4mm]
|\overrightarrow{OB}| &=& r_2 \\ \\[-4mm]
\angle O &=& \theta_2-\theta_1\end{array}$

Law of Cosines:

. . . $d^2\;=\;|AB|^2 \;=\;|OA|^2 + |OB|^2 - 2(|OA|)(|OB|)\cos(\angle O)$

Therefore: . $d^2 \;=\;r_1^2 + r_2^2 - 2(r_1)(r_2)\cos(\theta_2-\theta_1)$