Then the OM point is multiplied by ON=x 1x2+y 1y2.

Equation ax+by+c = 0, x2+y2 = 4.

Eliminate y: (a 2+b 2) x 2+2acx+(c 2-4a 2) = 0.

= = & gtx 1x2=(C^2-4A^2)/(A^2+B^2)

Similarly, if x is eliminated, you can get: y 1y2 = (c 2-4b 2)/(a 2+b 2).

x 1x2+y 1y2=(2c^2-4a^2-4b^2)/(a^2+b^2)

C2 = A2+B2 again, so: x 1x2+y 1y2=-2.

That is, OM vector multiplied by ON vector (o is the origin of coordinates) is equal to -2.