## Calculus 8th Edition

(a) $F_xG_x+F_yG_y+F_zG_z=0$ at $P$ (b) The two surfaces are orthogonal and their dot product is equal to zero.
a) Condition to be orthogonal: Two normal lines are known to be orthogonal at point P when $\nabla F \cdot \nabla G=0$ at point $P$ or, we can also say that their dot product must be zero. $\lt \dfrac{\partial F}{\partial x},\dfrac{\partial F}{\partial y},\dfrac{\partial F}{\partial z} \gt \cdot \lt \dfrac{\partial G}{\partial x},\dfrac{\partial G}{\partial y},\dfrac{\partial G}{\partial z} \gt =0$ Thus, $\lt \dfrac{\partial F}{\partial x}\dfrac{\partial G}{\partial x},\dfrac{\partial F}{\partial y}\dfrac{\partial G}{\partial y},\dfrac{\partial F}{\partial z} \dfrac{\partial G}{\partial z} \gt =0$ Hence, $F_xG_x+F_yG_y+F_zG_z=0$ at $P$ b) From the given data in the question, we have $\nabla P(x,y,z) \cdot \nabla Q(x,y,z)=\lt 2x,2y,-2z \gt \times \lt 2x,2y,2z \gt=4(x^2+y^2-z^2)$ or, $\nabla P(x,y,z) \cdot \nabla Q(x,y,z)=0$ Hence, we can see that the two surfaces are orthogonal and their dot product is zero.