Easiest one!!!

Algebra Level 1

x x and y y are real numbers such that x , y > 0 , x y = 1 x,y>0,\ xy=1 then calculate minimum value of x + y x+y


The answer is 2.

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4 solutions

Raj Rajput
Aug 1, 2015

Applying AM \ge GM as x , y > 0 x,\ y>0 and positive,

x + y 2 x y \frac{x+y}{2}\ge\sqrt{xy}

x + y 2 1 x y = 1 \implies\frac{x+y}{2}\ge 1\ \ \ \because xy=1

x + y 2 \implies x+y\ge 2

Minimum value of x + y x+y is 2 2

If we are using differentiation method, we are getting 2 & -2 as the extreme values of the equation, so why is 2 minimum ?

A Former Brilliant Member - 4 years, 1 month ago

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Because it is given in the question that x and y both are greater than 0 , therefore x+y cannot be -2 and thus answer results in +2 .

RAJ RAJPUT - 4 years, 1 month ago

Moreover we can only apply A.M>G.M when x and y both are positive numbers :)

RAJ RAJPUT - 4 years, 1 month ago
Mohammad Khaza
Jun 23, 2017

x,y is greater than 1.so only positive number which can give result 1 is (1multiplied by 1).

so 1+1=2.

Lukas Leibfried
Sep 17, 2015

A product will achieve the minimum sum of two factors if these two factors are equal to each other. In the case of all positive numbers, this can be achieved with two equal positive or two equal negative factors. However, because this problem specifies that x , y > 0 x,y>0 , we only need to find the sum of two equal positive factors. In the case of 1 1 , these factors are 1 1 and 1 1 , and 1 + 1 = 2 1+1=\boxed{2} .

This would be a great trick question if it weren't specified that x , y > 0 x,y>0 .

Galen Buhain
Feb 27, 2017

The only value of x and y is 1 because their product should be 1. 1 + 1 = 2

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