Verified answer

The normal vectors of the two planes are <8,8,8> and

<8,-8,8> respectively, or after dividing through by 8,

<1,1,1> and <1,-1,1>. So the angle theta between the

planes is such that

cos(theta) = (<1,1,1> dot <1,-1,1>) / (ll<1,1,1>ll*ll<1,-1,1>ll) =

(1 - 1 + 1) / (sqrt(3)*sqrt(3)) = 1/3.

Thus theta = cos^-1(1/3) = 70.529 degrees.

So I agree with you. I don't know what's going on. Could it be

a formatting issue or a significant figures issue?

These are parallel planes.