(a) $T = \frac{2u\sin\theta}{g} = 10\ Sec\ \Rightarrow u\sin\theta = 50\ m/s$ $\therefore H = \frac{u^{2}\sin^{2}\theta}{2g} = \frac{\left( u\sin\theta \right)^{2}}{2g} = \frac{50 \times 50}{2 \times 10} = 125\ m$
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Ph.D. in Physics with 19+ years of teaching experience.