( a 2 ( b 2 + 2 ) + 2 a b + 1 ) 2 = 6 ( a 2 b + a ) 2
Find the number of ordered pairs ( a , b ) that satisfy the equation above such that a , b are positive real numbers.
Bonus : Find number of real numbers that satisfy the relation given above!
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Well, for the bonus question, I guess the answer remains the same. Since you have just extended the set from positves to all reals(including negatives). What do you think @Harsh Shrivastava ?
Log in to reply
It would be good to do an additional check for the cases a = 0 , a b + 1 = 0 . (although it is easy)
Yep you are right! @Samarpit Swain
( a 2 ( b 2 + 2 ) + 2 a b + 1 ) 2 = 6 ( a 2 b + a ) 2 ⟹ ( a 2 b 2 + 2 a b + 1 + 2 a 2 ) 2 = 6 a 2 ( a b + 1 ) 2 ⟹ ( ( a b + 1 ) 2 + 2 a 2 ) 2 = 6 a 2 ( a b + 1 ) 2
Put x=ab+1 ⟹ ( x 2 + 2 a 2 ) 2 = 6 a 2 x 2 ⟹ x 4 − 2 a 2 x 2 + 4 a 4 = 0 ⟹ ( x 2 − a 2 ) 2 + 3 a 4 = 0 ⟹ ( ( a b + 1 ) 2 − a 2 ) 2 + ( 3 a 2 ) 2 = 0
Now sum of squares of two real numbers is zero means that they both must be equal to zero
⟹ 3 a 2 = ( ( a b + 1 ) 2 − a 2 ) = 0 ⟹ a = 0 ⟹ the equation ( a b + 1 ) 2 = a 2 becomes 1=0 which is a contradiction
So we get NO REAL solutions.
How would one think of creating this solution? What is the motivation / ideas behind such a approach?
Problem Loading...
Note Loading...
Set Loading...
Given the equation, ( a 2 ( b 2 + 2 ) + 2 a b + 1 ) 2 = 6 ( a 2 b + a ) 2 . Therefore, ( ( a b + 1 ) 2 + 2 a 2 ) = 6 a 2 ( a b + 1 ) 2 => ( a ( a b + 1 ) ( a b + 1 ) 2 + 2 a 2 ) 2 = 6 => ( a a b + 1 + a b + 1 2 a ) 2 = 6 .
Let us now make a substitution ( a a b + 1 ) = x . So our equation becomes: ( x + x 2 ) 2 = 6 => x 2 + x 2 4 − 2 = 0 = > x 4 − 2 x 2 + 4 = 0 (Since x = a a b + 1 = 0 ) .
Clearly the discriminant for this bi-quadratic equation is negative . Therefore, x 2 and further x satisfy non-real i.e imaginary values. Since x is an algebraic combination of positive reals a and b , it can never yield imaginary values. ( a a b + 1 ) comes out to be equal to 1 ± i 3
∴ Number of ordered pairs of ( a , b ) = 0 : )
Alternate solution: Since a , b are positive integers we can conveniently introduce AM-GM inequality for x: x ≥ 2 a a b + 1 × a b + 1 2 a = 2 2 ∴ x 2 ≥ 8 Since the minimum value of x 2 is 8 it can never attain the value of 6.
∴ Number of ordered pairs of ( a , b ) = 0 : )