111bccaabbc(b+c)ca(c+a)ab(a+b)
Applying R2→R2−R1 and R3→R3−R1:
=100bcca−bcab−bcbc(b+c)ca(c+a)−bc(b+c)ab(a+b)−bc(b+c)
For R2:
ca−bc=c(a−b)
ca(c+a)−bc(b+c)
=c(a2+ac−b2−bc)
=c[(a2−b2)+c(a−b)]
=c(a−b)(a+b+c)
For R3:
ab−bc=b(a−c)
ab(a+b)−bc(b+c)
=b(a2+ab−c2−bc)
=b[(a2−c2)+b(a−c)]
=b(a−c)(a+b+c)
=100bcc(a−b)b(a−c)bc(b+c)c(a−b)(a+b+c)b(a−c)(a+b+c)
Expanding along C1:
=1×c(a−b)b(a−c)c(a−b)(a+b+c)b(a−c)(a+b+c)
In this 2×2 matrix, C2=(a+b+c)×C1.
Since two columns are proportional, the determinant equals 0.