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# Question 1

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Could I please have some help on a few homework questions on quadratics? I'm having some trouble finding the answers.

1. The solution to the inequality $$y = -x^2 + ax + b \le 0$$ is $$(-\infty,-3] \cup [5,\infty).$$ Find the vertex of the parabola y = -x^2 + ax + b.

2. Find the largest constant C so that $$x^2 + y^2 + xy + 1 \ge C(x + y)$$ for all real numbers x and y.

3. Find all values of k so that the graphs of x^2 + y^2 = 4 + 12x + 6y and x^2 + y^2 = k + 4x + 12y intersect.

For the first question, I tried to write out the quadratic equation. For the second, I tried combining terms but I am not sure what to do with C. For the last, I tried making the two equations equal one another, but I'm not sure how to find all the values.

Any help will be appreciated, and thank you very much!

Mar 15, 2019

#1
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First one

The roots are x = - 3  and x = 5

So....the equation of the parabola is   y  = -  ( x - 5) (x + 3)

y = -x^2 + 2x - 15

Because of symmetry....the x coordinate of the vertex is   x = [ -3 + 5] / 2  = 2/2 = 1

So....to find the y coordinate we have

y = -(1)^2 + 2(1) + 15

y = -1 + 2 + 15

y = 16

So...the vertex is  (1, 16)

Here's a graph : https://www.desmos.com/calculator/5szp8sqydj   Mar 15, 2019
#2
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Thank you very much! Do you have any idea on how to approach the other two problems and solve them?

Guest Mar 16, 2019
#3
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Post your questions one at a time and seperately.

Think before you post the next and discuss what you have done (more thoroughly)

Plot them on the desmos calculator that CPhil has given you the address of.  Maybe then you can answer yourself.

Melody  Mar 17, 2019