Be careful with that domain!

Algebra Level 4

How many integer points does the domain of the given function contain?

f ( x ) = 1 { x + 1 } x 2 + 2 x \large f(x) = \dfrac{1}{\sqrt{ \{x+1\} - x^2 + 2x }}

Notation: { x } \{x\} denotes the fractional part function which is given by: { x } = x x \{x\} = x - \lfloor x \rfloor for x R x \in \mathbb{R} .

2 3 None 1

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2 solutions

For integral points, { x + 1 } = 0 \left \{x+1\right \} =0

So we must have, 2 x > x 2 x ( 2 x ) > 0 2x>x^2\implies x(2-x)>0 which is satisfied by x = 1 \boxed{x=1}

Md Zuhair
Apr 16, 2017

Here we go,

f ( x ) = 1 { x + 1 } x 2 + 2 x \large f(x) = \dfrac{1}{\sqrt{ \{x+1\} - x^2 + 2x }}

So we know { x + 1 } x 2 + 2 x > 0 \{x+1\} - x^2 + 2x > 0

\implies { x + 1 } > x 2 2 x \{x+1\} > x^2 - 2x

Again we know that,

\implies 0 { x + 1 } < 1 0 \leq \{x+1\} < 1

Hence it is clear that

\implies x 2 2 x < 1 x^2-2x<1

x 2 2 x + 1 < 2 \implies x^2-2x + 1 <2

( x 1 ) 2 < 2 \implies (x-1)^2<2

( x 1 ) 2 2 2 < 0 \implies (x-1)^2-\sqrt{2}^2<0

( x 1 2 ) ( x 1 + 2 < 0 \implies (x-1-\sqrt{2})(x-1+\sqrt{2}<0

So x ( 1 2 , 1 + 2 ) x \in (1-\sqrt{2} , 1+\sqrt{2})

Now see that we have x = 0 , 1 , 2 x= 0 , 1, 2 as integers

But by observation x = 0 x=0 and x = 2 x=2 makes f ( x ) f(x) undefined.

\implies Only integral x x which satisfies is x = 1 x=1

So 1 \boxed{1} integer points the domain of the given function contains.

Although your answer is correct, your domain isn't. You forgot some other conditions there. The domain actually is

x ( 3 13 2 , 0 ) ( 0 , 2 ) x \in \left( \dfrac{3-\sqrt{13}}{2} , 0 \right) \cup \left( 0 , 2 \right)

Tapas Mazumdar - 4 years, 1 month ago

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Can you point out where i made mistake?

Md Zuhair - 4 years, 1 month ago

The question can be solved graphically quite easily . Make the graph of x^2-2x and {x} . Notic the intersection points of the parabola and {x} graph.then we find that the domain is from ((3-√(13))/2,2)-{0}

Aditya Kumar - 4 years ago

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