Floor in the Function!

Algebra Level 3

Find the range of the following real valued function;

f ( x ) = 2 x 2 x \large f(x)=\dfrac{2^x}{2^{\lfloor x\rfloor}}

Notation : \lfloor \cdot \rfloor denotes the floor function .

[ 1 2 , 2 ] \left[\dfrac 12,2\right] ( 1 4 , 2 ] \left(\dfrac 14,2\right] ( 1 , 2 ] (1,2] ( 1 2 , 2 ] \left(\dfrac 12,2\right] [ 1 , 2 ) [1,2) [ 1 4 , 2 ) \left[\dfrac 14,2\right)

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3 solutions

Sravanth C.
May 27, 2016

We can rewrite the above function as;

f ( x ) = 2 x x = 2 { x } \large f(x)=2^{x-\lfloor x\rfloor}=2^{\{x\}}

Where { x } \{x\} denotes the fractional part of x x . Now since the range of the fractional part of x x is [ 0 , 1 ) [0,1) we can figure out the range of f ( x ) f(x) .

The minimum value of the function would be: f ( 0 ) = 2 0 = 1 f(0)=2^0=1 .

The maximum value of the function would be: f ( 0.999... ) = 2 0.999... 2 f(0.999...)=2^{0.999...}\to 2 .

That is the value tends to 2 but never quite reaches it. Thus the range of the function is [ 1 , 2 ) [1,2) , the open bracket denotes that it never equals 2.

Moderator note:

Simple standard approach.

f ( x ) = 2 x 2 x = 2 x 2 x { x } = 2 x 2 x 2 { x } = 2 { x } f(x)=\dfrac{2^x}{2^{\lfloor x \rfloor}}=\dfrac{2^x}{2^{x-\{x\}}}=\dfrac{2^x}{\frac{2^x}{2^{\{x\}}}}=2^{\{x\}} where {x} is the fractional part of x x .

As 0 { x } < 1 0 \le {\{x\}} <1 . The range of f ( x ) f(x) is [ 1 , 2 ) [1,2) .

Ashish Menon
May 27, 2016

x x can be written as x + y \left \lfloor x \right \rfloor + y where y y lies between 0 0 and 1 1 . It can be 0 0 when x x is an integer. But it can never be 1 1 . So, 2 x 2^{\left \lfloor x \right \rfloor} cancels from the numerator and the denominator. And what we are left is 2 y 2^y where 0 y < 1 0 \leq y < 1 . So minimum value is 2 0 = 1 2^0 = 1 . Maximum value is something less than 2 1 = 2 2^1 = 2 .

So, the answer is [ 1 , 2 ) \color{#69047E}{\boxed{\left[1 , 2\right)}} .

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