A creature on the moon throws a rock off a cliff. the height of the rock above the surface of the moon, in meters, t seconds after it is thrown is given by h(t) = -t^2 +8t +10. when does the rock first reach a height of 12 meters?
+99 t^2-8t-10=0
t^2-8t=10
t^2-8t+16=10+16
(t-4)^2=26
t-4=+(26^(1/2))
t=4+(26^(1/2))
t=9.1s
+2668 \(-t^2 + 8t + 10 = 12\)
\(-t^2 + 8t - 2 = 0\)
\(t = {-b \pm \sqrt{b^2-4ac} \over 2a}\)
\(t = {-8 \pm \sqrt{-8^2-(4 \times -1 \times -2)} \over 2 \times -1}\)
\(t= {-8 \pm \sqrt{56} \over -2}\)
\(t = {-8 \pm 2\sqrt{14} \over -2}\)
\(t = {4 \pm \sqrt{14}}\)
\(t = \color{brown}\boxed{4 - \sqrt{14}}\)
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