Question 1089529
{{{ f(x) = .24x^2 - .96x + 4 }}}
1989 = 0 
1998 = 9
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{{{ x[min] = -b/(2a) }}}
{{{ x[min] = .96/.48 }}}
{{{ x[min] = 2 }}}
1989 + 2 = 1991
The minimum population was in 1991
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Here's the plot:
{{{ graph( 400, 400, -1, 10,-2, 20, .24x^2 - .96x + 4 ) }}}