hi, I'm reading the book Mathematics for 3d game programming and computer graphics, and got a doubt in the mentioned section ( page 362 ). It says the distance between a point P ( center of sphere ) and L ( defined as < N, D > ) can be defined as

L*P = r.

Then the book says the relationship can also be written as

N*P + D = r.

Is this correct? Shouldn't it really be N*P - D = r? Thanks