In triangle $ABC$, $AB = 3$, $BC = 4$, $AC = 5$, and $BD$ is the angle bisector from vertex $B$.  If $BD = k \sqrt{2}$, then find $k$.
First, we should recognize that $\triangle ABC$ is a right triangle, since $3^2 + 4^2 = 5^2$. Yep, so $\angle ABC = 90^\circ$. Ok.  Let's also label point $D$ as the point at which $BD$ intersects $AC$. Sure.  Now let's look at $\triangle ABD$. Right.  We are given that $BD$ is an angle bisector. So $\angle ADB = \angle ABC / 2 = 45^\circ$. OOPS! Yes you are right. I meant to say $\angle ABD = \angle CBD = 45^\circ$. Also, we are given that $AB = 3$ and $BD = k\sqrt{2}$. So the segment $AD$ is 3/7 of the entire side $AC$. We know that $AC = 5$, so $AD = \frac{15}{7}$. Now we can use the Pythagorean Theorem on $\triangle ABD$. We know that $AB = 3$ and $BD = k\sqrt{2}$, and we just found that $AD = \frac{15}{7}$. Plugging in the numbers gives us $\left(\frac{15}{7}\right)^2 + (k\sqrt{2})^2 = 3^2$ Now we can solve for $k$.  $\left(\frac{15}{7}\right)^2 + (k\sqrt{2})^2 = 3^2$ simplifies to $\frac{225}{49} + 2k^2 = 9$. So $2k^2 = 9 - \frac{225}{49}$. Finally, we can divide by two to get $k^2 = \frac{108}{49}$ So $k = \pm\sqrt{\frac{108}{49}}$. The negative solution for $k$ doesn't make sense in this context, so we can ignore it. Right. So $k = \sqrt{\frac{108}{49}}$. The expression $\sqrt{\frac{108}{49}}$ can be simplified to $\sqrt{\frac{36}{49}} * \sqrt{3}$ Since $\sqrt{36} = 6$, and $\sqrt{49} = 7$, then $\sqrt{\frac{36}{49}} = \frac{6}{7}$. Thus, we have $k = \frac{6}{7} * \sqrt{3}$. But we can use the Law of Cosines to find the length of $BD$ The Law of Cosines is $BD^2 = AB^2 + AD^2 - 2(AB)(AD)cos\angle BAD$ Ok, so $\sin (\angle BAD) = \frac{BC}{AC} = \frac{4}{5}$ $\cos (\angle BAD) = \sqrt{1-\sin^2 \angle BAD} = \sqrt{1 - \frac{16}{25}} = \sqrt{\frac{9}{25}} = \frac{3}{5}$ Now we have everything we need to use the Law of Cosines. $BD^2 = AB^2 + AD^2 - 2(AB)(AD) \cos \angle BAD = 3^2 + \left(\frac{15}{7}\right)^2 - 2(3)\left(\frac{15}{7}\right)\left(\frac{3}{5}\right)$ Finally, $BD = \sqrt{\frac{288}{49}}$ $\sqrt{\frac{288}{49}} = \sqrt{\frac{144}{49} * 2} = \sqrt{\frac{144}{49}} * \sqrt{2} = \frac{12}{7} * \sqrt{2}$ Oh, so $k = \frac{12}{7}$. Right. $k = \frac{12}{7}$.

