Question

ON △ ABC with sides a, b, and c apply a2 - b2 = c2 - bc. The magnitude of angle A is?

#Maths Practice
# Trigonometry

Answer 1

[tex]\displaystyle \text{Rules of cosine}\\a^2=b^2+c^2-2bc\cos A\\a^2-b^2=c^2-2bc\cos A\\c^2-bc=c^2-2bc\cos A\\-bc=-2bc\cos A\\1=2\cos A\\\frac12=\cos A\\\boxed{\boxed{A=\frac13\pi\vee60^\circ}} [/tex]