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 Jun 4, 2015

Best Answer 

 #3
avatar+130584 
+13

Here's the algebraic solution without using polar coordinates.....

( x + 6)^2 + ( y - 9)^2 = 52

x^2 + y^2  = 13   →  y^2  = 13 - x^2  →  y = ± √(13 - x^2)

 

Let us first guess that y = the positive root of √(13 - x^2)

 

Substitute this into the first equation.........

 

(x + 6)^2 + ( √(13 - x^2) - 9)^2 = 52       expand

 

x^2 + 12x  + 36 + 13 - x^2 - 18 √(13 - x^2) + 81 = 52     simplify

 

12x - 18 √(13 - x^2) = -78   divide through by 6

 

2x - 3 √(13 - x^2) = -13   rearrange

 

2x + 13  = 3√(13 - x^2)    square both sides

 

4x^2 + 52x + 169  = 9(13 - x^2)    simplify

 

4x^2 + 52x +169 = 117 - 9x^2     rearrange

 

13x^2 + 52x + 52  = 0       divide through by 13

 

x^2 + 4x + 4 = 0   factor

 

(x + 2)^2 = 0   take the square root of both sides

 

x + 2 = 0     so  x =  -2   and  y = √(13 - x^2)  = √(13 - (-2)^2) = √(13 - 4) = √9 = 3

 

And we have seen above that (-2, 3) is a solution

 

----------------------------------------------------------------------------------------------------

Now...let us assume that the negative root of  √(13 - x^2)   also might work for y

 

The algebra looks eerily similar to what we did before...........

 

(x + 6)^2 + ( -√(13 - x^2) - 9)^2 = 52

 

x ^2 + 12x + 36  + 13 - x^2 + 18√(13 - x^2) + 81  = 52

 

12x + 18√(13 - x^2) = -78     

 

2x + 3√(13 - x^2) = -13

 

2x + 13  = -3√(13 - x^2)   

 

4x^2 + 52x + 169  = 9(13 - x^2)

 

4x^2 + 52x + 169 = 117 - 9x^2

 

13x^2 + 52x + 52 = 0

 

x^2 + 4x + 4 = 0

 

(x + 2)^2  = 0  

 

x + 2  = 0   ...  so....    x = -2   and y = -√(13 - x^2)  =- √(13 - (-2)^2) =- √(13 - 4) = -√9 = -3

 

However...notice the problem in the first equation if y = -3

 

(-2 + 6)^2 + (-3 - 9)^2  =  4^2 + (-12)^2  =  16 + 144 =  160  and this does not equal 52  !!!

 

So....we only have one solution.....

 

--------------------------------------------------------------------------------------------------

 

See???......I told you it was messy  !!!

 

Let us offer up our many sacrifices to the Altar of Desmos, The Spirit of the Graphing Calculator

 

 

 Jun 4, 2015
 #1
avatar+130584 
+10

( x + 6)^2 + ( y - 9)^2 = 52

x^2 + y^2  = 13  

 

The Algebra for this one could get a little messy......I might use a graphical approach....

 

https://www.desmos.com/calculator/cnkpzxaqrm

 

These are two circles that are tangent to each other....

 

The only "solution" point occurs at (-2, 3)....... And this is the point of tangency.......

 

 

 Jun 4, 2015
 #2
avatar+33669 
+10

Here's an algebraic approach (Chris is right - it's a little messy!):

simul eqns 1

 simul eqns 2

.

 Jun 4, 2015
 #3
avatar+130584 
+13
Best Answer

Here's the algebraic solution without using polar coordinates.....

( x + 6)^2 + ( y - 9)^2 = 52

x^2 + y^2  = 13   →  y^2  = 13 - x^2  →  y = ± √(13 - x^2)

 

Let us first guess that y = the positive root of √(13 - x^2)

 

Substitute this into the first equation.........

 

(x + 6)^2 + ( √(13 - x^2) - 9)^2 = 52       expand

 

x^2 + 12x  + 36 + 13 - x^2 - 18 √(13 - x^2) + 81 = 52     simplify

 

12x - 18 √(13 - x^2) = -78   divide through by 6

 

2x - 3 √(13 - x^2) = -13   rearrange

 

2x + 13  = 3√(13 - x^2)    square both sides

 

4x^2 + 52x + 169  = 9(13 - x^2)    simplify

 

4x^2 + 52x +169 = 117 - 9x^2     rearrange

 

13x^2 + 52x + 52  = 0       divide through by 13

 

x^2 + 4x + 4 = 0   factor

 

(x + 2)^2 = 0   take the square root of both sides

 

x + 2 = 0     so  x =  -2   and  y = √(13 - x^2)  = √(13 - (-2)^2) = √(13 - 4) = √9 = 3

 

And we have seen above that (-2, 3) is a solution

 

----------------------------------------------------------------------------------------------------

Now...let us assume that the negative root of  √(13 - x^2)   also might work for y

 

The algebra looks eerily similar to what we did before...........

 

(x + 6)^2 + ( -√(13 - x^2) - 9)^2 = 52

 

x ^2 + 12x + 36  + 13 - x^2 + 18√(13 - x^2) + 81  = 52

 

12x + 18√(13 - x^2) = -78     

 

2x + 3√(13 - x^2) = -13

 

2x + 13  = -3√(13 - x^2)   

 

4x^2 + 52x + 169  = 9(13 - x^2)

 

4x^2 + 52x + 169 = 117 - 9x^2

 

13x^2 + 52x + 52 = 0

 

x^2 + 4x + 4 = 0

 

(x + 2)^2  = 0  

 

x + 2  = 0   ...  so....    x = -2   and y = -√(13 - x^2)  =- √(13 - (-2)^2) =- √(13 - 4) = -√9 = -3

 

However...notice the problem in the first equation if y = -3

 

(-2 + 6)^2 + (-3 - 9)^2  =  4^2 + (-12)^2  =  16 + 144 =  160  and this does not equal 52  !!!

 

So....we only have one solution.....

 

--------------------------------------------------------------------------------------------------

 

See???......I told you it was messy  !!!

 

Let us offer up our many sacrifices to the Altar of Desmos, The Spirit of the Graphing Calculator

 

 

CPhill Jun 4, 2015

17 Online Users