AonZ
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
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|dw:1367018505450:dw|
AonZ
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@kropot72 @terenzreignz @Mertsj
Help pls
Btw i know angles D+B = 180
AonZ
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@hartnn help please!!!
Mertsj
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There is a formula for this. First let me relabel your picture so you will understand the formula better.
AonZ
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ok... only formula i know is
angle B + D=180
and
angle A+C = 180
Mertsj
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|dw:1367022862806:dw|
Mertsj
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\[\cos A=\frac{a^2+d^2-b^2-c^2}{2(ad+bc)}\]
Mertsj
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Just plug into the formula.
AonZ
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yes!!! that actually worked!!
Thank you so much
Mertsj
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yw
hartnn
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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
AonZ
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ohhhh i see