SOLUTION: Find all points on the circle x^2 + y^2 = 81 where the slope is 9/40. (x, y) = ( , ) (positive y-coordinate) (x, y) = ( , ) (negative y-coordinate) I've been trying everyt

Algebra ->  Graphs -> SOLUTION: Find all points on the circle x^2 + y^2 = 81 where the slope is 9/40. (x, y) = ( , ) (positive y-coordinate) (x, y) = ( , ) (negative y-coordinate) I've been trying everyt      Log On


   



Question 1157854: Find all points on the circle x^2 + y^2 = 81 where the slope is 9/40.
(x, y) = ( , ) (positive y-coordinate)
(x, y) = ( , ) (negative y-coordinate)
I've been trying everything and attempting to solve based on similar questions, please include all steps so I also know how to solve afterwards

Answer by Edwin McCravy(20054) About Me  (Show Source):
You can put this solution on YOUR website!
That means:

Find all points on the circle x^2 + y^2 = 81 where the slope OF A LINE DRAWN
TANGENT TO THE CIRCLE AT THAT POINT is 9/40.



x%5E2+%2B+y%5E2+=+81

We take the derivative implicitly

2x+%2B+2y%2Aexpr%28dy%2Fdx%29+=+0

Divide every term by 2

x+%2B+y%2Aexpr%28dy%2Fdx%29+=+0

Substitute 9/40 for (dy)/(dx)

x+%2B+y%2Aexpr%289%2F40%29+=+0

Solve for y:

y%2Aexpr%289%2F40%29+=+-x

Multiply both sides by 40/9

y+=+-40x%2F9

Substitute for y in the equation of the circle:

x%5E2+%2B+%28-40x%2F9%29%5E2+=+81

x%5E2+%2B+1600x%5E2%2F81+=+81

Multiply through by 81

81x%5E2+%2B+1600x%5E2+=+6561

1681x%5E2=6561

x%5E2=6561%2F1681

x=%22%22+%2B-+81%2F41%29

Substituting for x² in the equation of the circle:

x%5E2+%2B+y%5E2+=+81

6561%2F1681+%2B+y%5E2+=+81

Multiply through by 1681

6561%2B1681y%5E2=136161

1681y%5E2=129600

y%5E2=129600%2F1681

y=%22%22+%2B-+360%2F41

1681y%5E2=6480

y%5E2=6480%2F1681

y=%22%22+%2B-+360%2F41

So the two points where the slope of the tangent line is 9/40 are:

%28matrix%281%2C3%2C-81%2F41%2C%22%2C%22%2C360%2F41%29%29%29

and

%28matrix%281%2C3%2C81%2F41%2C%22%2C%22%2C-360%2F41%29%29%29

Edwin