Question 1089502
When the ball hits the ground, {{{ h=0 }}}, so
{{{ -4.9t^2 - 28t + 40 = 0 }}}
{{{ -49t^2 - 280t + 400 = 0 }}}
{{{ -7t^2 - 40t + 400/7 = 0 }}}
Use quadratic formula
{{{ t = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
{{{ a = -7 }}}
{{{ b = -40 }}}
{{{ c = 400/7 }}}
{{{ t = (-(-40) +- sqrt( (-40)^2-4*(-7)*(400/7) ))/(2*(-7)) }}} 
{{{ t = ( 40 +- sqrt( 1600 + 1600 ))/(-14) }}} 
{{{ t = ( 40 +- sqrt( 3200 ))/(-14) }}} 
{{{ t = ( 40 - 56.57 ) / (-14) }}}
{{{ t = 16.57/14 }}}
{{{ t = 1.18 }}}
The ball hits the ground in 1.2 sec
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