Question 326672
Plug in the number of feet given to find the time: {{{s =16t^2+32t}}} becomes {{{81feet =16t^2+32t}}}. Move the 81 to the right by subtracting 81 from both sides{{{0=16t^2+32t-81}}}. Use the quadratic equation with a=16, b=32, and c=-81: {{{t=(-b+-sqrt(b^2-4ac))/(2a)}}}. You find that t=1.4622seconds or t=-3.4622seconds. You know that the time can't be negative, so the answer is t=1.46seconds. To check you can plug 1.4622seconds in for t and make sure you get s=81feet =)
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