Precalculus help? The path of a ball is given by
y= -1/20x^2 +3x +5, where y is the height (in feet) of the ball and x is the horizontal distance (in feet) from where the ball was thrown. a. Find the maximum height of the ball.

Question

Precalculus help? The path of a ball is given by 
y= -1/20x^2 +3x +5, where y is the height (in feet) of the ball and x is the horizontal distance (in feet) from where the ball was thrown. a. Find the maximum height of the ball.

noelgreco · Answer

How do you find the coordinates of the vertex of a parabola?

anonymous · Answer

I assume I should complete the square to get it in the right form, but I'm trying and it's not really working out for me.

noelgreco · Answer

$$\frac{ -b }{ 2a }$$ gives you the x-coordinate of the vertex.

anonymous · Answer

So should I end up with -30?

anonymous · Answer

Oops, positive 30.

noelgreco · Answer

yes, now that's the point directly beneath the ball at its highest point.  How can you use that to find the height?

anonymous · Answer

Now can I just plug 30 into the equation and solve for y?

noelgreco · Answer

Yep.

anonymous · Answer

Thanks so much, I really appreciate it. :)

noelgreco · Answer

You're welcome.  That formula for the x-coordinate of the vertex will come in handy this year.  It's really just the part of the quadratic formula outside the radical.

Have fun.