Will it even end?

Algebra Level 4

1 x 1 4 x 2 + 4 x 3 1 x 4 < 1 30 \dfrac{1}{x-1}-\dfrac{4}{x-2}+\dfrac{4}{x-3}-\dfrac{1}{x-4} < \dfrac{1}{30}

If the range of x x satisfying the above inequality is in the form as given below.

( , a ) ( b , c ) ( d , e ) ( f , g ) ( h , ) (-\infty,a)\cup (b,c)\cup (d,e) \cup (f,g) \cup (h,\infty)

Find a + b + c + d + e + f + g + h a+b+c+d+e+f+g+h .


The answer is 20.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Rishabh Jain
Jun 26, 2016

Relevant wiki: Polynomial Inequalities Problem Solving - Basic

( 4 x 3 4 x 2 ) + ( 1 x 1 1 x 4 ) < 1 30 \left(\dfrac{4}{x-3}-\dfrac{4}{x-2}\right)+\left(\dfrac{1}{x-1}-\dfrac{1}{x-4} \right)< \dfrac{1}{30} 4 ( x 2 ) ( x 3 ) ( x 2 5 x + 4 ) + 2 3 ( x 1 ) ( x 4 ) x 2 5 x + 4 < 1 30 \implies \dfrac{4}{\underbrace{(x-2)(x-3)}_{(\color{#D61F06}{x^2-5x+4})+2}}-\dfrac{3}{\underbrace{(x-1)(x-4)}_{\color{#D61F06}{x^2-5x+4}}}<\dfrac1{30}

Put x 2 5 x + 4 = t \color{#D61F06}{x^2-5x+4}=t

4 t + 2 3 t < 1 30 \implies \dfrac4{t+2}-\dfrac3{t}<\dfrac1{30} t 6 t ( t + 2 ) < 1 30 \implies \dfrac{t-6}{t(t+2)}<\dfrac1{30} t 2 28 t + 180 t ( t + 2 ) > 0 \implies \dfrac{t^2-28t+180}{t(t+2)}>0 ( t 10 ) ( t 18 ) t ( t + 2 ) > 0 \implies \dfrac{(t-10)(t-18)}{t(t+2)}>0 Put t t back and factorise further to obtain:

( x + 1 ) ( x + 2 ) ( x 6 ) ( x 7 ) ( x 1 ) ( x 4 ) ( x 2 ) ( x 3 ) > 0 \implies \dfrac{(x+1)(x+2)(x-6)(x-7)}{(x-1)(x-4)\left(x-2\right)\left(x-3\right)}>0

Plot a number line to see solution set as:-

x ( , 2 ) ( 1 , 1 ) ( 2 , 3 ) ( 4 , 6 ) ( 7 , ) x\in(-\infty,-2)\cup(-1,1)\cup\left(2,3\right)\cup(4,6)\cup(7,\infty)

Hence,

2 1 + 1 + 2 + 3 + 4 + 6 + 7 = 20 -2-1+1+2+3+4+6+7=\boxed{20}

Typo after the line you put back t: t + 2 = ( x 2 ) ( x 3 ) t+2=(x-2)(x-3)

Akshat Sharda - 4 years, 11 months ago

Log in to reply

Typo... Corrected

Rishabh Jain - 4 years, 11 months ago

Was too lazy to factorize :p Anyway, good solution. Could also be explained using the Wavy curve method though

Mehul Arora - 4 years, 11 months ago

Log in to reply

Drawing the number line is exactly the same.... Curve is drawn in mind to plot sign scheme on number line.... :-)

Rishabh Jain - 4 years, 11 months ago
Wouter Fokkema
Jun 26, 2016

Call the given function f ( x ) f(x) .

Notice that f ( x ) = f ( 5 x ) f(x)=f(5-x) , which gives h = 5 a h = 5 - a , g = 5 b g = 5 - b , f = 5 c f = 5 - c and e = 5 d e = 5 - d .

So a + b + c + d + e + f + g + h = ( a + h ) + ( b + g ) + ( c + f ) + ( d + e ) = 5 + 5 + 5 + 5 = 20 a + b + c + d + e + f + g + h = (a + h) + ( b + g) + (c + f) + (d + e) = 5 + 5 + 5 + 5 = \boxed{20}

Oh, that's a nice trick. You have made use of the symmetries in the problem statement to find the answer without finding the actual range of the function. I have upvoted your solution (+1)

Congratulations on writing your first solution on Brilliant, and thanks for sharing your approach :)

Pranshu Gaba - 4 years, 11 months ago

The exact same method for me too.

Josh Banister - 4 years, 11 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...