=4R where R-circumradius; a=2R‌sin‌A,b=2R‌sin‌B,c=2R‌sin‌C] ⇒4R2(sin2A+sin2B+sin2C)=8R2 ⇒sin2A+sin2B+sin2C=2 ⇒(cos‌2‌A+cos‌2‌B+cos‌2‌C)=−1 ⇒−1−4‌cos‌A‌cos‌B‌cos‌C=−1 ⇒cos‌A‌cos‌B‌cos‌C=0 So, any one among A,B and C has to be