⎩ ⎨ ⎧ a + a 2 + b 2 a + 8 b = 2 b + a 2 + b 2 8 a − b = 0
If ( a 1 , b 1 ) , ( a 2 , b 2 ) , … , ( a n , b n ) are all the real solutions of ( a , b ) which satisfy the system of equations above, find the value of m = 1 ∏ n a m b m .
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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 + b 2 a 2 + 8 a b = 2 a
and
b 2 + a 2 + b 2 8 a b − b 2 = 0
Solving by subtracting the first equation from the second and simplifying yields ( a − 1 ) 2 = b 2
Expanding and moving stuff around should give you ( a − b ) ( a + b ) + 1 = 2 a 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 2 + b 2 a b + 8 b 2 = 2 b
and
a b + a 2 + b 2 8 a 2 − a b = 0
Solving by adding the equations together and simplifying yields b = 1 − a 4
When you plug this into ( a − b ) ( a + b ) + 1 = 2 a , you get an equation which simplifies into
a 4 − 4 a 3 + 6 a 2 − 4 a − 1 5 = 0
Factoring gives you ( a − 3 ) ( a + 1 ) ( a 2 − 2 a + 5 ) = 0
Plugging in a for the real solutions gives you (3,-2) and (-1,2)
3 × ( − 2 ) × ( − 1 ) × 2 = 1 2