We have,
Applying C1 \(\longrightarrow\)C1 - C3 and C2 \(\longrightarrow\)C2 - C3
Taking common (cos A - cos C) from C1 and (cos B - cos C) from C2, we get
Applying C1 \(\longrightarrow\)C1 - C2, we get
Expanding along R1, we get
Hence, ΔABC is an isosceles triangle.