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

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AonZ
 3 years ago
Best ResponseYou've already chosen the best response.0@kropot72 @terenzreignz @Mertsj Help pls Btw i know angles D+B = 180

Mertsj
 3 years ago
Best ResponseYou've already chosen the best response.2There is a formula for this. First let me relabel your picture so you will understand the formula better.

AonZ
 3 years ago
Best ResponseYou've already chosen the best response.0ok... only formula i know is angle B + D=180 and angle A+C = 180

Mertsj
 3 years ago
Best ResponseYou've already chosen the best response.2\[\cos A=\frac{a^2+d^2b^2c^2}{2(ad+bc)}\]

Mertsj
 3 years ago
Best ResponseYou've already chosen the best response.2Just plug into the formula.

AonZ
 3 years ago
Best ResponseYou've already chosen the best response.0yes!!! that actually worked!! Thank you so much

hartnn
 3 years ago
Best ResponseYou've already chosen the best response.1did 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
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