Let the set R be the set of real numbers.
Let r = [ ( x , y ) ∈ R × R ∣ 1 6 x 4 + 1 6 y 2 + 5 = 8 x 2 + 1 6 y ]
Find the number of possible real co-ordinate(s) of ( x , y ) which is a member of r .
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Hint : 1 6 x 4 + 1 6 y 2 + 5 = 8 x 2 + 1 6 y can be written as
( 4 x 2 − 1 ) 2 + ( 4 y − 2 ) 2 = 0
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From the given question, for coordinates ( x , y )
1 6 x 4 − 8 x 2 + 1 6 y 2 − 1 6 y + 5 = 0
Let a = 1 6 y 2 − 1 6 y + 5 ........(1)
Hence, the equation becomes,
1 6 x 4 − 8 x 2 + a = 0 ......(2)
The roots of the above equation exists only if the discriminant of this equation is positive
Hence, ( 8 ) 2 − ( 4 ) ( 1 6 ) ( a ) ≥ 0
∴ a ≤ 1
∴ 1 6 y 2 − 1 6 y + 5 ≤ 1
∴ 1 6 y 2 − 1 6 y + 4 ≤ 0
∴ 4 y 2 − 4 y + 1 ≤ 0
∴ ( 2 y − 1 ) 2 ≤ 0
But square of any value is a positive value
Hence, 2 y − 1 = 0 ⟹ y = 2 1
Putting the above value of y in (1), we get,
∴ a = 1
Putting a = 1 in (2) we get
1 6 x 4 − 8 x 2 + 1 = 0
∴ ( 2 x − 1 ) 2 ( 2 x + 1 ) 2 = 0 ⟹ x = ± 2 1
Hence, there are only two order pairs ( 2 1 , 2 1 ) and ( − 2 1 , 2 1 ) which satisfies this equation
Hence the answer is 2