Atypical equations

Algebra Level 2

Two positive numbers have the property that the sum of their squares is 30 and the sum of the reciprocals is 2 what is their product?


The answer is 3.

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2 solutions

Alex Zhong
Apr 4, 2015

Since 1 x + 1 y = 2 x + y x y = 2 x + y = 2 x y . \dfrac{1}{x}+\dfrac{1}{y}= 2 \implies \dfrac{x+y}{xy} = 2 \implies x+y = 2xy.

Since x 2 + y 2 = 30 x 2 + 2 x y + y 2 = 30 + 2 x y ( x + y ) 2 = 30 + 2 x y . x^2 + y^2 = 30 \implies x^2+2xy + y^2 = 30 +2xy \implies (x+y)^2 = 30+2xy.

Thus, we have ( 2 x y ) 2 = 30 + 2 x y . (2xy)^2 = 30 + 2xy.

Substitute w = x y , w = xy, then 4 w 2 2 w 30 = 0 2 ( w 3 ) ( 2 w + 5 ) = 0. 4w^2 -2w -30 = 0 \implies 2(w-3)(2w+5) = 0.

For positive numbers, we have x y = w = 3 . xy=w = \boxed{3}.

Aran Pasupathy
May 17, 2015

Let the two positive numbers be x and y.

x²+y²= 30 -----------(1a)

1/x + 1/y= 2---------(2a)

For (1a), (x+y)²= x²+2xy+y²

So (x+y)²-2xy= 30-----(1b)

For (2a), establishing a common denominator of xy, we get (x+y)/xy= 2------(2b)

So (x+y)= 2xy and, substituting this into (1b), we get (x+y)²-(x+y)= 30

So (x+y)²-(x+y)-30= 0.

Substituting (x+y) with u, we get u²-u-30= 0.

Factorising this, we get (u-6)(u+5)= 0.

So u= 6 or -5, implying that (x+y)= 6 or -5.

Since (x+y)= 2xy, xy= (x+y)/2.

If (x+y)= 6, then xy= 6/2= 3.

If (x+y) = -5, then xy= -5/2.

Since only integers are accepted as the answer, xy= 3.

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