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 #2
avatar+130511 
+10

I think this is supposed to be :

2x^2-72xy+23y^2-80x-60y-125=0

 

We need to first calculate this:

cot2Θ = [A - C]  / B   = [2 - 23]/[-72]  = [-21]/[-72]  = [7/24]

 

And, from trig, we have that cos2Θ  = 7/25

And using trig again, we can caclulate  sinΘ  and cosΘ, thusly

 sinΘ  = √[(1 - cos2Θ)/2 ] = √[(1 - 7/25)/2] = √[9/25] = 3/5

cosΘ =  √[(1 + cos2Θ)/2 ] = √[(1 + 7/25)/2] = √[16/25] = 4/5

 

Now....we need to make the following substitutions

x = cosΘ X  -  sinΘ Y      and y =  sinΘ X + cosΘ Y    ....  or, just .......

x = (4/5)X - (3/5)Y       and  y = (3/5)X + (4/5)Y

 

So we have

2[(4/5)X - (3/5)Y]^2-72[ (4/5)X - (3/5)Y][(3/5)X + (4/5)Y]+23[(3/5)X + (4/5)Y]^2-80[ (4/5)X - (3/5)Y]-60[ (3/5)X + (4/5)Y -125=0-80[ (4/5)X - (3/5)Y]  

 

And term by term, we have.......

[(32/25)X^2  - (48/25)XY + (18/25)Y^2] +

[(-864/25)X^2 - (504/25)XY + (864/25)Y^2] +

[(207/25)X^2 +(552/25)XY + (368/25)Y^2]  +

[48Y  - 64X]  +

[-36X  - 48Y ] -

125    =  0

 

Simplifying the above, we get

50Y^2  - 25X^2 -100X  - 125 = 0     divide through by 25

2Y^2 - x^2 - 4x - 5 = 0   complete the square on x  

2Y^2 - [x^2 + 4x + 4  - 4] = 5

2Y^2 - (x +2)^2 = 1

 

And we have the recognizable equation of a hyperbola.

 

Here's a picture of both the original  graph with the xy term and the "normal" graph without rotation...

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

 

BTW...the original graph's rotation can be found thusly:

cot2Θ = [A - C]  / B   = [2 - 23]/[-72]  = [-21]/[-72]  = [7/24] 

cot-1(7/24) =73.739795291688°

So half of this is the angle of rotation ≈ 36.87° 

 

  

Apr 24, 2015
 #2
avatar+118723 
+5
Apr 24, 2015

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