Questiona. Find $z_{t t}$ for $$z_{t t}=\quad z(x, y, t)=\cos (\sqrt{4+49 t}) \cdot \sin (2 x) \cdot \sin (7 y)$$ $a^{b}$ $\sin (a) \quad \frac{\partial}{\partial x} f$ : $\infty$ $\alpha$ $\Omega$ ? b. Does $$u=\sin (\sqrt{53} \cdot t) \cdot \sin (2 x) \cdot \sin (7 y)$$ satisfy the membrane equation $$u_{t t}=u_{x x}+u_{y y} ?$$ Yes No