Easy Inequality......

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Find the solution set for ( x + 5 ) ( 2 x 1 ) 3 \dfrac {\left( x+5\right) }{\left( 2x-1\right) }\leq3 . Note :- Careful, scope for silly mistakes !

(-infinity, -1.5] U [0.5, infinity) [1.6, infinity) (-infinity, 1.6] (-infinity, 0.5) U [1.6, infinity)

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2 solutions

x + 5 2 x 1 3 x + 5 3 ( 2 x 1 ) 2 x 1 0 5 x + 8 2 x 1 0 \begin{array}{l} \displaystyle\frac{{x + 5}}{{2x - 1}} \le 3 \\ \Leftrightarrow \displaystyle\frac{{x + 5 - 3(2x - 1)}}{{2x - 1}} \le 0 \\ \Leftrightarrow \displaystyle\frac{{ - 5x + 8}}{{2x - 1}} \le 0 \\ \end{array}

We have this table:

\begin{array}{*{20}{c}} \hline & x & & {\left( { - \infty ,\displaystyle\frac{1}{2}} \right)} & {\displaystyle\frac{1}{2}} & {\left( {\displaystyle\frac{1}{2},\displaystyle\frac{8}{5}} \right)} & {\displaystyle\frac{8}{5}} & {\left( {\displaystyle\frac{8}{5}, + \infty } \right)} & \\ \hline & { - 5x + 8} & & + & + & + & 0 & - & \\ \hline & {2x - 1} & & - & 0 & + & + & + & \\ \hline & {\displaystyle\frac{{ - 5x + 8}}{{2x - 1}}} & & - & {||} & + & 0 & - & \\ \hline \end{array}

As shown in this table, the solution set is ( , 1 2 ) [ 8 5 , + ) \left( { - \infty ,\displaystyle\frac{1}{2}} \right) \cup \left[ {\displaystyle\frac{8}{5}, + \infty } \right)

Nice technique !

Venkata Karthik Bandaru - 6 years, 2 months ago

Observe that when the denominator is an algebraic expression, we need to consider 2 cases :- denominator positive and denominator negative. After considering both cases, we need to take the most appropriate solution set of the variable that satisfy both cases.

You have not included 0.5 0.5 in the correct option. I think it should be included.

Soumo Mukherjee - 6 years, 3 months ago

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At 0.5, the expression on left hand side of the given inequality becomes undefined !

Venkata Karthik Bandaru - 6 years, 2 months ago

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Yes, I had my doubt clarified by Abhineet.

Thanks.

Your solution picture is titled. Straightening it would provide a better view.

:)

Soumo Mukherjee - 6 years, 2 months ago

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@Soumo Mukherjee Actually when I upload any pic to brilliant, the pic is getting tilted. Did you mean titled or tilted ?

Venkata Karthik Bandaru - 6 years, 2 months ago

No, actually not...Look, if you put x = 0.5 x=0.5 , then the denominator tends to infinity which is not accepted...and as a result 0.5 0.5 is not taken as a solution.

A Former Brilliant Member - 6 years, 3 months ago

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yeah, I overlooked that. Actually I used method of interval. So the transformed expression had an extraneous root: 0.5 0.5

thanks :)

Soumo Mukherjee - 6 years, 3 months ago

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@Soumo Mukherjee Oh okay...well, no problem!:) anytime...

A Former Brilliant Member - 6 years, 3 months ago

Sorry guys, I dont know latex, except the very basics and so I couldnt write the whole solution in latex. Is the picture clear ?

Venkata Karthik Bandaru - 6 years, 2 months ago

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