The equations of the motion of a stone
"x(t)=20\\cos 30^{\\circ}\\cdot t\\\\ y(t)=45+20\\sin30^{\\circ}\\cdot t-5t^2"
(a) When a stone strikes the ground "y=0," so
Root
(b) The final speed of a stone
"v=\\sqrt{(v_0\\cos \\theta)^2+(v_0\\sin \\theta-gt)^2}"
"v=\\sqrt{(20\\cos30^{\\circ})^2+(20\\sin 30^{\\circ}-10\\times 4.16)^2}"
"=36\\:\\rm m\/s"
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