Difficult system of equations

Algebra Level 4

{ a + a + 8 b a 2 + b 2 = 2 b + 8 a b a 2 + b 2 = 0 \large { \begin{cases} a + \frac{a+8b}{a^2+b^2} = 2 \\ b + \frac{8a-b}{a^2+b^2} = 0 \end{cases} }

If ( a 1 , b 1 ) , ( a 2 , b 2 ) , , ( a n , b n ) (a_1,b_1), (a_2,b_2) , \ldots ,(a_n,b_n) are all the real solutions of ( a , b ) (a,b) which satisfy the system of equations above, find the value of m = 1 n a m b m \displaystyle \prod_{m=1}^n a_m b_m .


The answer is 12.

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1 solution

Brian Wang
Nov 11, 2015

You start by multiplying both sides of the first equation by a and both sides of the second equation by b to get
a 2 + a 2 + 8 a b a 2 + b 2 = 2 a { a }^{ 2 }+\frac { { a }^{ 2 }+8ab }{ { a }^{ 2 }+{ b }^{ 2 } } \quad =\quad 2a
and
b 2 + 8 a b b 2 a 2 + b 2 = 0 b^{ 2 }+\frac { 8ab-{ b }^{ 2 } }{ { a }^{ 2 }+{ b }^{ 2 } } \quad =\quad 0
Solving by subtracting the first equation from the second and simplifying yields ( a 1 ) 2 = b 2 { (a-1) }^{ 2 }\quad =\quad { b }^{ 2 }
Expanding and moving stuff around should give you ( a b ) ( a + b ) + 1 = 2 a (a-b)(a+b)+1\quad =\quad 2a Then, you multiply both sides of the first equation by b, and both sides of the second equation by a to get
a b + a b + 8 b 2 a 2 + b 2 = 2 b ab+\frac { ab+8{ b }^{ 2 } }{ { a }^{ 2 }+{ b }^{ 2 } } \quad =\quad 2b
and
a b + 8 a 2 a b a 2 + b 2 = 0 ab+\frac { { 8a }^{ 2 }-ab }{ { a }^{ 2 }+{ b }^{ 2 } } \quad =\quad 0
Solving by adding the equations together and simplifying yields b = 4 1 a b\quad =\quad \frac { 4 }{ 1-a }
When you plug this into ( a b ) ( a + b ) + 1 = 2 a (a-b)(a+b)+1\quad =\quad 2a , you get an equation which simplifies into
a 4 4 a 3 + 6 a 2 4 a 15 = 0 { a }^{ 4 }-4{ a }^{ 3 }+6{ a }^{ 2 }-4a-15\quad =\quad 0
Factoring gives you ( a 3 ) ( a + 1 ) ( a 2 2 a + 5 ) = 0 (a-3)(a+1)({ a }^{ 2 }-2a+5)\quad =\quad 0
Plugging in a for the real solutions gives you (3,-2) and (-1,2)
3 × ( 2 ) × ( 1 ) × 2 = 12 3\times (-2)\times (-1)\times 2\quad =\quad 12


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