It is given that the matrix [α21aα1b11c] is singular, so
∣α21aα1b11c∣=0
⇒α2(c−b)−α(c−a)+(b−a)=0
Above equation holds true if α=1. Now,
(b−a)(c−b)(a−c)2+(a−c)(c−b)(b−a)2+(a−c)(b−a)(c−b)2
=(a−b)(b−c)(c−a)(a−b)3+(b−c)3+(c−a)3
=(a−b)(b−c)(c−a)3(a−b)(b−c)(c−a)=3
[∵x3+y3+z3=3xyzifx+y+z=0]