Integers and Inequality

Algebra Level 2

How many integers x x satisfy the inequality x 2 81 x 2 36 x < 0 ? \frac{|x^2-81|}{x^2-36x}<0?

38 36 32 34

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3 solutions

Mike Paul
Dec 1, 2016

Since absolute values are always non-negative, we know that the numerator will always be non-negative. Therefore, for the expression to be negative, the denominator must be negative and the numerator must be strictly positive. Factoring the denominator gives us

x 2 36 x = x ( x 36 ) x^2-36x = x(x-36)

The zeroes of this expression are 0 and 36.

If x > 36 x>36 , both factors x x and x 36 x-36 are positive, and their product is positive.

If x < 0 x<0 , both factors are negative, and their product is positive.

If 0 < x < 36 0<x<36 , the factor x x is positive and the factor x 36 x-36 is negative, and their product is negative.

So we've narrowed down the possible solutions to the integers from 1 to 35 inclusive. Now we just need to determine if any of these integers make the numerator zero. Solving the equation x 2 81 = 0 x^2-81=0 for x x , we find that the numerator is zero when x = ± 9 x=\pm 9 . Therefore, the solutions are all of the integers from 1 to 35 inclusive except 9, so there are 34 integer values of x x that satisfy the inequality.

mind blowing

Dilip Kumar - 4 years, 6 months ago

32 is also between 1to 35 and less than 36 so why it doesn't be the answer.

Krunal Deshmukh - 4 years, 4 months ago

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It is all the numbers between 0 and 36, except 9.

Marta Reece - 3 years, 2 months ago
Shivamani Patil
Jun 27, 2014

If x is equal to any negative integer then result will be positive.

If x=0 we get undefined result. If x=9 then results is 0.And also x=36

we get undefined results so we have numbers from 1 to 35 except 9.

Are there no more systematic formulations of this solution?

Joshua Nesseth - 4 years, 7 months ago
Rayhan Emon
Mar 6, 2014

if x=9 then x2-81=0 and if x=36 then x2-36x=0. So result is 34.

The problem could be even better if 35 was one of the options

Marta Reece - 3 years, 2 months ago

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