9. (10 points) Find $(a,b)$ with $-\dfrac{1}{2} \le b \le \dfrac{1}{2}$ so that

• The point $P=(1,a,b)$ is on the surface $E$ defined by

$$\frac{x}{2}-\frac{y}{4}+\frac{\sin(2z)}{4}=0$$

• The tangent plane to $E$ at $P$ contains lines

$$l_1(t)=P+t\left(\frac{1}{2},1,0\right)$$ $$l_2(t)=P+t(0,2,1)$$