1. The Rules of
Sine

2. A
b
c
B a C

3. How about for
scalene triangle ?

4. C
Height line is the line from an angle of triangle
b a that is perpendicular with opposite side.
h
A c D B

5. C Look at the triangle beside!
By the same way, we get :
a
b
A D c B
So, we get :
It is called by the rule of sine.
This rules can be used for cases :
• Two angles and any one side length are given
• Two sides and an angle of opposite side are given

6. Example
Determine the size of angle and the length of sides that are not
given from the triangle below :
B
5 cm
A C

7. Answer

8. Activities Class
1. Determine the undetermined elements from triangle of KLM :
M
K 25 cm L
Given : the angle of K is 400 and the angle of L is 550.
2. Desi and Vita are standing on the edge of a river and are separated as
far as 40 m. across the river, Ardi is seen to be standing and looking at
both the girls. If the angle formed by Ardi-Desi-Vita is 650 and the angle
formed by Ardi-Vita-Desi is 500. Determine the distance of Ardi and Desi
and the distance of Ardi from Vita!