Mathematics - Properties of Triangle Question with Solution | TestHub

Write r_1=$\Delta$/(s-a), r_2=$\Delta$/(s-b), r_3=$\Delta$/(s-c), cancel ${\Delta }^2$ and simplify to (b-a)(c-a)=2(s-a)². Since 2s=a+b+c, this becomes $b^2$+$c^2$=$a^2$. Thus A=90° by the cosine rule.