APPENDIX D - Trigonometry - D Exercises - Page A 33: 88

$A=\frac{1}{2}absin\theta$

The area of a triangle is given by $A=\frac{1}{2}\times Base\times Height$ Here, base =b and height =h In order to find height, we will consider $sin\theta=\frac{opp}{hyp}=\frac{h}{b}$ $h=bsin\theta$ Thus, $A=\frac{1}{2}\times a\times bsin\theta$ Hence, $A=\frac{1}{2}absin\theta$