Verified answer

Answer:

As 0≤cos

−1

θ≤π

For cos

−1

x+cos

−1

y+cos

−1

z=3π this to true

⇒cos

−1

x=cos

−1

y=cos

−1

z=π

⇒x=y=z=−1

∴∑xy=xy+yz+zx=(−1)

2

+(−1)

2

+(−1)

2

=3