1. Solve for all missing angles and sides:
Warm UP!
Solve for all missing angles and sides: Z 5 3 Y x

2. What formulas did you use to solve the triangle?
Pythagorean theorem
SOHCAHTOA
All angles add up to 180o in a triangle

3. Could you use those formulas on this triangle?
Solve for all missing angles and sides:
This is an oblique triangle. An oblique triangle is any non-right triangle.
35o 3 5 y z x
There are formulas to solve oblique triangles just like there are for right triangles!

4. Solving Oblique Triangles Laws of Sines and Cosines & Triangle Area
Students will solve trigonometric equations both graphically and algebraically.
d. Apply the law of sines and the law of cosines.

5. General Comments A B C a b c
You have learned to solve right triangles. Now we will solve oblique triangles (non-right triangles).
Note: Angles are Capital letters and the side opposite is the same letter in lower case. C a b
Make sure students understand the opposite side and the 2 adjacent sides (vs. adjacent and hypotenuse). A c B

6. What we already know The interior angles total 180.
We can’t use the Pythagorean Theorem. Why not?
For later, area = ½ bh
Larger angles are across from longer sides and vice versa.
The sum of two smaller sides must be greater than the third. A B C a b c

7. sin A = sin B = sin C a b c sin A sin B sin C
The Law of Sines helps you solve for sides or angles in an oblique triangle.
sin A = sin B = sin C
a b c
(You can also use it upside-down)
a = b = c
sin A sin B sin C

8. Use Law of SINES when ... AAS - 2 angles and 1 adjacent side
…you have 3 parts of a triangle and you need to find the other 3 parts.
They cannot be just ANY 3 dimensions though, or you won’t have enough info to solve the Law of Sines equation.
Use the Law of Sines if you are given:
AAS - 2 angles and 1 adjacent side
ASA - 2 angles and their included side
SSA – (SOMETIMES) 2 sides and their adjacent angle

9. General Process for Law Of Sines
Except for the ASA triangle, you will always have enough information for 1 full fraction and half of another. Start with that to find a fourth piece of data.
Once you know 2 angles, you can subtract from 180 to find the 3rd.
To avoid rounding error, use given data instead of computed data whenever possible.

10. Example 1 Solve this triangle:
The angles in a ∆ total 180°, so solve for angle C.
Set up the Law of Sines to find side b: A C B
70°
80° 12 c b
Angle C =
Side b =
Side c =

11. Example 2: Solve this triangleB C a b c
You’re given both pieces for sinA/a and part of sinB/b, so we start there. 45 50
=30 85

12. Example 3: Solve this triangleB C a b c
Since we can’t start one of the fractions, we’ll start by finding C.
C = 180 – 35 – 10 = 135
135
36.5
11.1 35 10 45
Since the angles were exact, this isn’t a rounded value. We use sinC/c as our starting fraction.
Check for calculator ability
Using your calculator

13. You try! Solve this triangleC B
115°
30°
a = 30 c b

14. The Law of Cosines
When solving an oblique triangle, using one of three available equations utilizing the cosine of an angle is handy. The equations are as follows:

15. General Strategies for Using the Law of Cosines
The formula for the Law of Cosines makes use of three sides and the angle opposite one of those sides. We can use the Law of Cosines:
SAS - two sides and the included angle
SSS - all three sides

16. Example 1: Solve this triangle
Now, since we know the measure of one angle and the length of the side opposite it, we can use the Law of Sines.
87.0°
15.0
17.0 c B A
Use the relationship:
c2 = a2 + b2 – 2ab cos C
c2 = – 2(15)(17)cos(87°)
c2 = …
c = 22.1

17. Example 2: Solve this triangle
31.4
23.2
38.6 C
We start by finding cos A.

18. You TRY: Solve a triangle with a = 8, b =10, and c = 12.
Solve a triangle with A = 88o, B =16o, and c = 14.
A = 41.4o a = 8
B = 55.8o b = 10
C = 82.8o c = 12
A = 88o a = 12.4
B = 16o b = 3.4
C = 76o c = 14