An algebra problem by milind prabhu

Algebra Level 2

Find the number of non-zero real solutions to the equation x = x . -\sqrt { x } =x.


The answer is 0.

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5 solutions

Hung Woei Neoh
Jul 16, 2016

x = x x + x = 0 x ( x + 1 ) = 0 -\sqrt{x}=x\\ x+\sqrt{x}=0\\ \sqrt{x}\left(\sqrt{x}+1\right)=0

x = 0 x = 0 \sqrt{x}=0 \implies x=0

x + 1 = 0 x = 1 \sqrt{x}+1=0 \implies \sqrt{x}=-1 \implies No real solution

Therefore, the only real solution of this equation is x = 0 x=0 . There are 0 \boxed{0} non-zero real solutions for this equation

Beautiful one!

Ραμών Αδάλια - 4 years, 11 months ago

But can't sqrt(1)=-1, and thus -sqrt(1)=1?

Matthew K - 4 years, 11 months ago

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No, 1 1 , 1 1 \sqrt{1}\neq -1,-\sqrt{1}\neq1 .

We define x \sqrt{x} as the principal square root function, this symbol represents the positive square root of x x . x \sqrt{x} only yields two different results: either it is a non-negative number (for all x 0 x\geq0 ), or it is a complex number (for all x < 0 x<0 ). x \sqrt{x} can never give you a negative number

Hung Woei Neoh - 4 years, 11 months ago
Milind Prabhu
Jul 16, 2016

For the left hand to be defined, x x has to be positive. The x \sqrt { x } is always positive or zero. Here it cannot be zero so it is positive. So the left hand side is always negative. The right hand side on the other hand is always positive as x x is always positive. So there exists no non zero real solution to the equation.

Moderator note:

Good approach of considering the domains and its implication.

I made the same approach!

Arthur Hertz - 4 years, 9 months ago
Goh Choon Aik
Jul 16, 2016

For any x x , its squared counterpart is always positive. Therefore, the answer is that there are 0 values that makes the equation true.

However, a more correct solution would be infinity because the question specified 'non-zero real number'. Since the square root of, let's say, 4 can be both 2 and - 2, the equation would be true for the negative root.

Note that x 2 = x \sqrt {x^2}=|x|

milind prabhu - 4 years, 11 months ago

x 0 x = x 0 On the other hand: x 0 x 0 x 0 , x 0 x = 0 Therefore x = 0 is the only real solution to this equation \color{#D61F06}{\boxed{\sqrt{x}\geq 0}}\implies \color{#3D99F6}{\boxed{-\sqrt{x}=x\leq 0}}\\ \text{On the other hand:}\\ \color{#D61F06}{\sqrt{x}\geq 0}\implies \color{#20A900}{\boxed{x\geq 0}}\\ \color{#3D99F6}{x\leq 0},\color{#20A900}{x\geq 0}\implies \color{teal}{\boxed{\boxed{x=0}}}\\ \text{Therefore} \;x=0\;\text{is the only real solution to this equation}

Loo Soo Yong
Jul 19, 2016

Another way is to sketch the graph. Clearly there are only one point of intersection (which is at the origin), and hence the number of non-zero solutions is 0.

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