Alternatively, you can draw a perpendicular line from A to meet BC to set up the equation using Pythagoras theorem now that the two right angled triangles have common height. Those who talk of cosine rule doesn't appreciate the beauty of maths, imo.
Using *cosine rule* cos A = (3^2 + 5^2 - 7^2) / (2 x 3 x 5) = (9 + 25 - 49 = -15) / (2 x 15) = -1 / 2 = cos 120 (since 49 > 9 + 25 an obtuse angle was anyway expected). So, A = 120 deg.
![Pythagoras Would Be Proud: High School Students' New Proof of the Pythagorean Theorem [TRIGONOMETRY]](http://i.ytimg.com/vi/p6j2nZKwf20/mqdefault.jpg)
Excellent video man 😊 👍
Alternatively, you can draw a perpendicular line from A to meet BC to set up the equation using Pythagoras theorem now that the two right angled triangles have common height.
Those who talk of cosine rule doesn't appreciate the beauty of maths, imo.
nice vid keep up
Using *cosine rule* cos A = (3^2 + 5^2 - 7^2) / (2 x 3 x 5) = (9 + 25 - 49 = -15) / (2 x 15) = -1 / 2 = cos 120 (since 49 > 9 + 25 an obtuse angle was anyway expected).
So, A = 120 deg.
There is no necessity to find the measure of angle CAD
Angle BAC =Angle ACD + angle ADC = 30 +90=120 degrees
just use law of cosine
Coulda just used the law os cosines)
But elementary proof are nice😊