SOLUTION: What are the coordinates of the maximum (highest) point on the graph of 4x ^2 + y ^ 2 + 8x -2y - 3 = 0

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Question 1065506: What are the coordinates of the maximum (highest) point on the graph of 4x ^2 + y ^ 2 + 8x -2y - 3 = 0
Answer by Edwin McCravy(20060) About Me  (Show Source):
You can put this solution on YOUR website!
This depends on whether you are still in algebra or in calculus.
I'll do it both ways:

1.  Using calculus:

By implicit differentiation:

4x%5E2+%2B+y%5E2+%2B+8x-2y+-+3+=+0

8x%2B2y%2Aexpr%28%28dy%29%2F%28dx%29%29+%2B+8+-+2%2Aexpr%28%28dy%29%2F%28dx%29%29+=+0

We set %28dy%29%2F%28dx%29 equal to 0

8x+%2B+8+=+0

8x=-8

x=-1, substituting:

4%28-1%29%5E2+%2B+y%5E2+%2B+8%28-1%29-2y+-+3+=+0

4%2By%5E2-8-2y-3=0
y%5E2-2y-7=0

y+=+%282+%2B-+sqrt%284%2B28%29+%29%2F2+
y+=+%282+%2B-+sqrt%2832%29+%29%2F2+
y+=+%282+%2B-+sqrt%2816%2A2%29+%29%2F2+
y+=+%282+%2B-+4%2Asqrt%282%29+%29%2F2+
y+=+%282%281+%2B-+sqrt%282%29%29%29%2F2+
y+=+1+%2B-+sqrt%282%29+

Answer, since the + gives the larger value, 
the maximum point is %28matrix%281%2C3%2C-1%2C%22%2C%22%2C1%2Bsqrt%282%29%29%29

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2.  If you're in algebra,

4x%5E2+%2B+y%5E2+%2B+8x-2y+-+3+=+0

Swap the middle two terms to get like lettered terms together:

4x%5E2+%2B+8x+%2B+y%5E2+-+2y+-+3+=+0

Add 3 to both sides:

4x%5E2+%2B+8x+%2B+y%5E2+-+2y+=+3+

Factor 4 out of the first two terms:

4%28x%5E2+%2B+2x%29+%2B+y%5E2+-+2y+=+3+

Complete the square inside the parentheses:
                2%2Aexpr%281%2F2%29=1
                1%5E2=1
                
Add +1 inside the parentheses and add +4*1 to the right side
since 4 times what is added inside the parentheses is added
to the left side and must also be added to the right side.

4%28x%5E2+%2B+2x+%2B+1%29+%2B+y%5E2+-+2y+=+3%2B4+

Complete the square on the y's:
                -2%2Aexpr%281%2F2%29=-1
                %28-1%29%5E2=1

Add +1 to both sides:

4%28x%5E2+%2B+2x+%2B+1%29+%2B+y%5E2+-+2y%2B1+=+3%2B4%2B1+

Factor the two trinomials, which are perfect squares, and
write them as the squares of binomials, combine numers
on the right side:

4%28x%2B1%29%5E2+%2B+%28y-1%29%5E2+=+8+

Divide through by 8

4%28x%2B1%29%5E2%2F8+%2B+%28y-1%29%5E2%2F8+=+8%2F8+

%28x%2B1%29%5E2%2F2+%2B+%28y-1%29%5E2%2F8+=+1+

Compare to the standard equation for an ellipse:

%28x-h%29%5E2%2Fb%5E2+%2B+%28y-h%29%5E2%2Fa%5E2+=+1+ since a > b

center (h,k) = (-1,1), a=sqrt%288%29=sqrt%284%2A2%29=2sqrt%282%29, b=sqrt%282%29

Graph:



The highest point on the ellipse is the upper vertex, which has the
same x-coordinate as the center, which is -1,

The y-coordinate of the upper vertex, which is a=sqrt%282%29 units 
above the center, which has y-coordinate 1, so we add a=sqrt%282%29 to
1 and get 1%2Bsqrt%282%29.

Thus the highest point on the ellipse is:

%28matrix%281%2C3%2C-1%2C%22%2C%22%2C1%2Bsqrt%282%29%29%29

Edwin