putting this in the determinant form we get
a.a a.b a.c
b.a b.b b.c =X2
c.a c.b c.c
1 1/2 1/2
1/2 1 1/2 =X2
1/2 1/2 1
we get X2=2/4
X=1/√2
putting this in the determinant form we get
a.a a.b a.c
b.a b.b b.c =X2
c.a c.b c.c
1 1/2 1/2
1/2 1 1/2 =X2
1/2 1/2 1
we get X2=2/4
X=1/√2