An algebra problem by Prem Chebrolu

Algebra Level 3

Let p p and q q be distinct real numbers, such that 2005 + p = q 2 2005 + p = q^{2} and 2005 + q = p 2 2005 + q = p^{2} . Find the value of ( 2014 + p q 2014 + pq ).


The answer is 10.

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2 solutions

Zico Quintina
Jun 27, 2018

If we subtract the given equations, we get

p 2 q 2 = q p ( p + q ) ( p q ) = - ( p q ) p + q = - 1 ( 1 ) [ p , q are distinct, so p q 0 ] p 2 + 2 p q + q 2 = 1 ( 2 ) \begin{array}{rlrrrl} p^2 - q^2 &= \ \ q - p \\ (p + q)(p - q) &= \ \ \text{-}(p - q) \\ p + q &= \ \ \text{-}1 & & \small (1) & & \small [ \ p, q \text{ are distinct, so } p - q \ne 0 \ ] \\ p^2 + 2pq + q^2 &= \ \ 1 & & \small (2) \end{array}

Now add the original equations:

p 2 + q 2 = p + q + 4010 p 2 + q 2 = 4009 ( 3 ) [ Substituting ( 1 ) ] 2 p q = - 4008 [ Subtracting ( 2 ) ( 3 ) ] p q = - 2004 \begin{array}{rlrrrl} p^2 + q^2 &= \ \ p + q + 4010 \\ p^2 + q^2 &= \ \ 4009 & & \small (3) & & \small \text{[ Substituting } (1) \ ] \\ 2pq &= \ \ \text{-}4008 & & & & \small \text{[ Subtracting } (2) - (3) \ ] \\ pq &= \ \ \text{-}2004 \end{array}

so our answer is p q + 2014 = 10 pq + 2014 = \boxed{10}

We can write this system as

q 2 p = 2005 , p 2 q = 2005 p 2 q = q 2 p p 2 q 2 = q p ( p q ) ( p + q ) = ( p q ) q^{2} - p = 2005, p^{2} - q = 2005 \Longrightarrow p^{2} - q = q^{2} - p \Longrightarrow p^{2} - q^{2} = q - p \Longrightarrow (p - q)(p + q) = -(p - q) .

Now since p , q p,q must be distinct we must have that p + q = 1 q = ( 1 + p ) p + q = -1 \Longrightarrow q = -(1 + p) . Substituting this result into 2005 + p = q 2 2005 + p = q^{2} yields

2005 + p = ( ( 1 + p ) ) 2 = 1 + 2 p + p 2 p 2 + p 2004 = 0 2005 + p = (-(1 + p))^{2} = 1 + 2p + p^{2} \Longrightarrow p^{2} + p - 2004 = 0 . Similarly we can find that q 2 + q 2004 = 0 q^{2} + q - 2004 = 0 .

Thus p , q p,q are the two roots of the equation x 2 + x 2004 = 0 x^{2} + x - 2004 = 0 , and so by Vieta's we have that p q = 2004 pq = -2004 , and so 2014 + p q = 2014 2004 = 10 2014 + pq = 2014 - 2004 = \boxed{10} .

Note that these roots p , q p,q are 1 ± 8017 2 \dfrac{-1 \pm \sqrt{8017}}{2} .

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