kPA•kPB=(y-y1)(y+y1)/[(x-x1)(x+x1)]=(y^2-y1^2)/(x^2-x1^2)=[(b^2x^2/a^2-b^2)-(b^2x1^2/a^2-b^2)]/(x^2-x1^2)=(b^2/a^2)(x^2-x1^2)/(x^2-x1^2)=b^2/a^2
