Let p and q be distinct real numbers, such that 2 0 0 5 + p = q 2 and 2 0 0 5 + q = p 2 . Find the value of ( 2 0 1 4 + p q ).
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We can write this system as
q 2 − p = 2 0 0 5 , p 2 − q = 2 0 0 5 ⟹ p 2 − q = q 2 − p ⟹ p 2 − q 2 = q − p ⟹ ( p − q ) ( p + q ) = − ( p − q ) .
Now since p , q must be distinct we must have that p + q = − 1 ⟹ q = − ( 1 + p ) . Substituting this result into 2 0 0 5 + p = q 2 yields
2 0 0 5 + p = ( − ( 1 + p ) ) 2 = 1 + 2 p + p 2 ⟹ p 2 + p − 2 0 0 4 = 0 . Similarly we can find that q 2 + q − 2 0 0 4 = 0 .
Thus p , q are the two roots of the equation x 2 + x − 2 0 0 4 = 0 , and so by Vieta's we have that p q = − 2 0 0 4 , and so 2 0 1 4 + p q = 2 0 1 4 − 2 0 0 4 = 1 0 .
Note that these roots p , q are 2 − 1 ± 8 0 1 7 .
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If we subtract the given equations, we get
p 2 − q 2 ( p + q ) ( p − q ) p + q p 2 + 2 p q + q 2 = q − p = - ( p − q ) = - 1 = 1 ( 1 ) ( 2 ) [ p , q are distinct, so p − q = 0 ]
Now add the original equations:
p 2 + q 2 p 2 + q 2 2 p q p q = p + q + 4 0 1 0 = 4 0 0 9 = - 4 0 0 8 = - 2 0 0 4 ( 3 ) [ Substituting ( 1 ) ] [ Subtracting ( 2 ) − ( 3 ) ]
so our answer is p q + 2 0 1 4 = 1 0