# Math Help - triangle

1. ## triangle

dear...
i dun knw how to solve this pls help? and anyone who can recommend me a gud site to study Geometry? thank you so much!

2. Originally Posted by janeyandtaz
dear...
i dun knw how to solve this pls help? and anyone who can recommend me a gud site to study Geometry? thank you so much!
I don't know what exactly you want to solve ...

If you want
- to calculate the length of side AC then use Cosine rule
- to calculate the value of the 2 missing angles then use Cosine rule again
- the value of the area of the triangle ABC then use the trigonometric formula of the area a:

$a_{triangle} = \left\{\begin{array}{l}\frac12 \cdot AC \cdot AB \cdot \sin(A) \\
\frac12 \cdot BC \cdot AC \cdot \sin(C) \\
\frac12 \cdot AB \cdot BC \cdot \sin(B) \end{array}\right.$

3. oh m sorry, i forgot to type...how is it to find the area of the triangle.

4. Originally Posted by janeyandtaz
oh m sorry, i forgot to type...how is it to find the area of the triangle.
Probably you have found the value of the area by using the first equation of the formulae I gave you in my previous post:

$a_{triangle}=\dfrac12 \cdot AB \cdot AC \cdot \sin(A)$

Plug in the values of AB, AC and angle A.

I've got $5\sqrt{2}\approx 7.07$

5. thank you, earboth
but i still dont understand how to count AC. cant we use Pythagoras Formula? or i assume Pythagoras is only for right triangle?
i am so confuse

6. Originally Posted by janeyandtaz
thank you, earboth
but i still dont understand how to count AC. cant we use Pythagoras Formula? or i assume Pythagoras is only for right triangle?
i am so confuse
I'm sorry I used the wrong formula.

You know the sides BC and BA and the angle B. So pick the appropriate formula:

$a_{triangle} = \dfrac12 \cdot BC \cdot BA \cdot \sin(B)$

which will yield the result I posted in my previous answer.

7. Originally Posted by janeyandtaz
thank you, earboth
but i still dont understand how to count AC. cant we use Pythagoras Formula? or i assume Pythagoras is only for right triangle?
i am so confuse
Pythagoras' Theorem can only be used for right triangles. If you've been given questions about triangles that are not right triangles then you certainly must have been taught the sine rule, cosine rule as well as the area rule. I suggest you go back to your class notes or textbook and thoroughly review this material.

8. thank you