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a^2 + b^2 = 20 ab + 6b = 32 what are all the pairs (a; b)

 Apr 22, 2015

Best Answer 

 #2
avatar+130477 
+10

 

ab + 6b = 32    →  b(a + 6)  = 32  →   b = 32/(a + 6)   (2)

Substitue (2) into (1) and we  have

a^2 + [32/(a + 6)]^2  = 20      simplify

a^2(a + 6)^2 + 1024 = 20(a+6)^2

a^2(a^2 + 12a + 36) + 1024 = 20a^2 + 240a + 720

a^4 + 12a^3 + 36a^2 + 1024 = 20a^2 + 240a + 720

a^4 + 12a^3 + 16a^2 - 240a + 304  = 0

Using the on site solver we have one real soution for a

 

a4+12×a3+16×a2240×a+304=0{a=2×3×i8a=2×3×i8a=2}{a=83.4641016151377546ia=8+3.4641016151377546ia=2}

And b =

32 /(a + 6)   = 32 /(2 + 6)  =  32 /8   = 4

 

  

 Apr 22, 2015
 #1
avatar+33658 
+10

You can do this on the calculator here using the "Equation" mode:

 Input

 

 Output

 

.

 Apr 22, 2015
 #2
avatar+130477 
+10
Best Answer

 

ab + 6b = 32    →  b(a + 6)  = 32  →   b = 32/(a + 6)   (2)

Substitue (2) into (1) and we  have

a^2 + [32/(a + 6)]^2  = 20      simplify

a^2(a + 6)^2 + 1024 = 20(a+6)^2

a^2(a^2 + 12a + 36) + 1024 = 20a^2 + 240a + 720

a^4 + 12a^3 + 36a^2 + 1024 = 20a^2 + 240a + 720

a^4 + 12a^3 + 16a^2 - 240a + 304  = 0

Using the on site solver we have one real soution for a

 

a4+12×a3+16×a2240×a+304=0{a=2×3×i8a=2×3×i8a=2}{a=83.4641016151377546ia=8+3.4641016151377546ia=2}

And b =

32 /(a + 6)   = 32 /(2 + 6)  =  32 /8   = 4

 

  

CPhill Apr 22, 2015
 #3
avatar+118703 
0

Thanks Alan and CPhill    

A very nice display from each of you.:)

 Apr 23, 2015

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