## AonZ Group Title HELP PLEASE!!! ABCD is a cyclic quadrilateral in which AB=5. BC=6. CD=7. AD=8. show that cos ADC = 13/43 one year ago one year ago

1. AonZ

|dw:1367018505450:dw|

2. AonZ

@kropot72 @terenzreignz @Mertsj Help pls Btw i know angles D+B = 180

3. AonZ

4. Mertsj

There is a formula for this. First let me relabel your picture so you will understand the formula better.

5. AonZ

ok... only formula i know is angle B + D=180 and angle A+C = 180

6. Mertsj

|dw:1367022862806:dw|

7. Mertsj

$\cos A=\frac{a^2+d^2-b^2-c^2}{2(ad+bc)}$

8. Mertsj

Just plug into the formula.

9. AonZ

yes!!! that actually worked!! Thank you so much

10. Mertsj

yw

11. hartnn

did you get how how found cos A ? he just applied cosine rule, twice. once in triangle ABD and then in triangle BCD, also knowing A+C = 180, so cos C = -cos A

12. AonZ

ohhhh i see