Can you minimize?

Calculus Level 5

f ( x ) = 0 1 t t x d t f(x) = \int_{0}^{1} t|t-x| \ dt

If the minimum value of f ( x ) f(x) above, where x R x \in \mathbb R , is ω \omega , what is the value of 1000 ω \lfloor{1000 \omega} \rfloor ?


The answer is 97.

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2 solutions

Prakhar Gupta
Feb 10, 2015

The given integral is:- f ( x ) = 0 1 t t x d t f(x) = \int_{0}^{1} t|t-x|dt This can be broken into two parts. (Here we assume that x x lies between 0 0 and 1 1 ):- f ( x ) = 0 x t ( x t ) d t + x 1 t ( t x ) d t f(x) = \int_{0}^{x} t(x-t) dt + \int_{x}^{1} t(t-x)dt Solving the above integral we get:- f ( x ) = x 3 3 x 2 + 1 3 f(x) = \dfrac{x^{3}}{3} -\dfrac{x}{2} + \dfrac{1}{3} Differentiating w.r.t. x x :- f ( x ) = x 2 1 2 f'(x) = x^{2}- \dfrac{1}{2} For minima, f ( x ) = 0 f'(x)=0 x 2 = 1 2 x^{2}=\dfrac{1}{2} x = 1 2 x=\dfrac{1}{\sqrt{2}} Hence, f ( x ) = 1 6 2 1 2 2 + 1 3 f(x) = \dfrac{1}{6\sqrt{2}} -\dfrac{1}{2\sqrt{2}} + \dfrac{1}{3} Hence, f ( x ) = 2 2 6 f(x)= \dfrac{2-\sqrt{2}}{6} Therefore:- ω = 0.097631 \omega = 0.097631

How can we assume that x lies between 0 and 1? Doesn't t lie in that interval?

Aravindh Shankar - 6 years, 3 months ago

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Take the case of x x lying outside the interval. The value of integral turns out to be less than the answer.

Vikram Waradpande - 6 years, 3 months ago
Chew-Seong Cheong
Aug 21, 2019

Note that f ( x ) = 0 1 t t x d t = { 0 1 t ( t x ) d t for x < t 0 1 t ( x t ) d t for x t f(x) = \displaystyle \int_0^1 t|t-x|\ dt = \begin{cases} \displaystyle \int_0^1 t (t-x) \ dt & \text{for }x < t \\ \displaystyle \int_0^1 t (x-t) \ dt & \text{for }x \ge t \end{cases}

For x 0 x \le 0 , f ( x ) = 0 1 t ( t x ) d t = 1 3 x 2 f(x) = \displaystyle \int_0^1 t(t-x) \ dt = \frac 13 - \frac x2 min ( f ( x ) ) = f ( 0 ) = 1 3 \implies \min (f(x)) = f(0) = \dfrac 13 .

For x 0 x \ge 0 , f ( x ) = 0 1 t ( x t ) d t = x 2 1 3 f(x) = \displaystyle \int_0^1 t(x-t) \ dt = \frac x2 - \frac 13 min ( f ( x ) ) = f ( 1 ) = 1 6 \implies \min (f(x)) = f(1) = \dfrac 16 .

For 0 < x < 1 0 < x < 1 ,

f ( x ) = 0 1 t t x d t = 0 x t ( x t ) d t t < x + x 1 t ( t x ) d t t > x = x 3 2 x 3 3 + 1 3 x 2 x 3 3 + x 3 2 = x 3 3 x 2 + 1 3 f ( x ) = x 2 1 2 Equating to 0 x = 1 2 Since 0 < x < 1 min ( f ( x ) ) = f ( 1 2 ) = 2 2 6 0.097631 Since f ( 1 2 ) > 0 \begin{aligned} f(x) & = \int_0^1 t |t-x| \ dt \\ & = \underbrace{\int_0^x t (x-t) \ dt}_{t < x} + \underbrace{\int_x^1 t (t-x) \ dt}_{t> x} \\ & = \frac {x^3}2 - \frac {x^3}3 + \frac 13 - \frac x2 - \frac {x^3}3 + \frac {x^3}2 \\ & = \frac {x^3}3 - \frac x2 + \frac 13 \\ \implies f'(x) & = x^2 - \frac 12 & \small \color{#3D99F6} \text{Equating to }0 \\ x & = \frac 1{\sqrt 2} & \small \color{#3D99F6} \text{Since }0 < x < 1 \\ \implies \min(f(x)) & = f \left(\frac 1{\sqrt 2}\right) = \frac {2-\sqrt 2}6 \approx 0.097631 & \small \color{#3D99F6} \text{Since }f'' \left(\frac 1{\sqrt 2}\right) > 0 \end{aligned}

Therefore, 1000 ω = 97 \lfloor 1000\omega \rfloor = \boxed{97} .

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