Sine formula
![\sin (angle)=\frac{\text{opposite side}}{hypotenuse}](https://img.qammunity.org/2023/formulas/mathematics/college/gfio7ht2rqnpmjj88h853oj3ouqqhxfasx.png)
Considering angle C from triangle BCD, the opposite side is side BD and the hypotenuse is side BC which length is a units. Then:
![\begin{gathered} \sin (\angle C)=(BD)/(a) \\ \text{ Isolating BD} \\ \sin (\angle C)\cdot a=BD \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/lxux65novc3ewu5g07653c0du2ejsj12lj.png)
The area of a triangle is calculated as follows:
![A=(1)/(2)\cdot\text{base}\cdot\text{height}](https://img.qammunity.org/2023/formulas/mathematics/college/p3ifjlsloehmw66rm7tsz2h9tw7qjsnoq3.png)
In triangle ABC the base is b units long and its height is segment BD, then the area of triangle ABC is:
![\begin{gathered} A=(1)/(2)\cdot b\cdot BD \\ \text{ Substituting with the previous result:} \\ A=(1)/(2)\cdot b\cdot a\cdot\sin (\angle C) \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/zi7aiudw58e646n40iwljxr1uajgggq8fo.png)