Fung-sion

Algebra Level 4

Let the set R R be the set of real numbers.

Let r = [ ( x , y ) R × R 16 x 4 + 16 y 2 + 5 = 8 x 2 + 16 y ] r = [(x, y) \in R \times R | 16x^{4} + 16y^{2} + 5 = 8x^{2} + 16y]

Find the number of possible real co-ordinate(s) of ( x , y ) (x, y) which is a member of r r .


The answer is 2.

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2 solutions

Tilak Patel
Apr 18, 2014

From the given question, for coordinates ( x , y ) (x,y)

16 x 4 8 x 2 + 16 y 2 16 y + 5 = 0 16x^{4} - 8x^{2} + 16y^{2} - 16y +5 = 0

Let a = 16 y 2 16 y + 5 a = 16y^{2} - 16y + 5 ........(1)

Hence, the equation becomes,

16 x 4 8 x 2 + a = 0 16x^{4} - 8x^{2} + a = 0 ......(2)

The roots of the above equation exists only if the discriminant of this equation is positive

Hence, ( 8 ) 2 ( 4 ) ( 16 ) ( a ) 0 (8)^{2} - (4)(16)(a) \geq 0

a 1 \therefore a \leq 1

16 y 2 16 y + 5 1 \therefore 16y^{2} - 16y + 5 \leq 1

16 y 2 16 y + 4 0 \therefore16y^{2} - 16y + 4 \leq 0

4 y 2 4 y + 1 0 \therefore 4y^{2} - 4y + 1 \leq 0

( 2 y 1 ) 2 0 \therefore(2y - 1)^{2} \leq 0

But square of any value is a positive value

Hence, 2 y 1 = 0 y = 1 2 2y - 1 = 0 \implies y = \frac{1}{2}

Putting the above value of y y in (1), we get,

a = 1 \therefore a = 1

Putting a = 1 a = 1 in (2) we get

16 x 4 8 x 2 + 1 = 0 16x^{4} - 8x^{2} + 1 = 0

( 2 x 1 ) 2 ( 2 x + 1 ) 2 = 0 x = ± 1 2 \therefore(2x - 1)^{2}(2x + 1)^{2} = 0 \implies x = \pm \frac{1}{2}

Hence, there are only two order pairs ( 1 2 , 1 2 ) (\frac{1}{2} , \frac{1}{2}) and ( 1 2 , 1 2 ) (- \frac{1}{2} , \frac{1}{2}) which satisfies this equation

Hence the answer is 2 \boxed{2}

Hint : 16 x 4 + 16 y 2 + 5 = 8 x 2 + 16 y 16x^{4} + 16y^{2} + 5 = 8x^{2} + 16y can be written as

( 4 x 2 1 ) 2 + ( 4 y 2 ) 2 = 0 (4x^{2} - 1)^{2} + (4y - 2)^{2} = 0

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