تمرين جميل

Prove that a triangle ABC is right-angled if and only if
sin^2A+sin^2B+sin^2C=2

ملاحظة استاذنا شحات جامع
في السطر السادس

Cos(a-b)=cosc implies a-b=c ora-b=-c
So a=b+c or b=a+c
Then a is right angle or b is right angle
The nthe triangle abc is a right triangle at a or b or c