ACT Math : How to find the solution of a rational equation with a binomial denominator

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #1 : Binomial Denominators

For the equation \displaystyle x^{2}-10x+21=0, what is(are) the solution(s) for \displaystyle x?

Possible Answers:

\displaystyle 2,5

\displaystyle -7

\displaystyle 3,7

\displaystyle -3

\displaystyle -3,-7

Correct answer:

\displaystyle 3,7

Explanation:

\displaystyle x^{2}-10x+21=0, can be factored to (x -7)(x-3) = 0. Therefore, x-7 = 0 and x-3 = 0. Solving for x in both cases, gives 7 and 3. 

Example Question #52 : Variables

Simplify:

\displaystyle \frac{x^2+3x+4x+12}{x+4}

Possible Answers:

\displaystyle x-4

\displaystyle \frac{x^2+7x+12}{x+4}

\displaystyle x-3

\displaystyle x+4

\displaystyle x+3

Correct answer:

\displaystyle x+3

Explanation:

In order to begin this kind of a problem, it's key to look at parts of the rational expression that can be simplified.

In this case, the denominator is an already-simplified binomial; however, the numerator can be factored through "factoring by grouping." This can be a helpful idea to keep in mind when you come across a polynomial with four terms and simplifying is involved.

\displaystyle {x^2+3x+4x+12} can be simplified first by removing the common factor of \displaystyle x from the first two terms and the common factor of \displaystyle 4 from the last two terms:

\displaystyle x(x+3)+4(x+3)

This leaves two terms that are identical \displaystyle (x+3) and their coefficients, which can be combined into another term to complete the factoring:

\displaystyle (x+4)(x+3)

Consider the denominator; the quantity \displaystyle (x+4) appears, so the \displaystyle (x+4) in the numerator and in the denominator can be cancelled out. The simplified expression is then left as \displaystyle (x+3).

Example Question #51 : Variables

Simplify:

\displaystyle \frac{x^2+9x-10}{x-1}

Possible Answers:

\displaystyle (x+1)

\displaystyle (x-10)

\displaystyle (x+2)

\displaystyle (x+10)

\displaystyle (x-1)

Correct answer:

\displaystyle (x+10)

Explanation:

In order to begin this kind of a problem, it's key to look at parts of the rational expression that can be simplified. 

In this case, the denominator is an already-simplified binomial; however, the numerator can be factored. 

\displaystyle x^2+9x-10

The roots will be numbers that sum up to \displaystyle 9 but have the product of \displaystyle -10.

The options include:

\displaystyle (-1)(10) = -10
\displaystyle (1)(-10) = -10
\displaystyle (2)(-5) = -10
\displaystyle (-2)(5)= -10

When these options are summed up:

\displaystyle -1+10=9
\displaystyle 1-10=-9
\displaystyle 2-5=-3
\displaystyle -2+5=3

We can negate the last three options because the first option of \displaystyle -1 and \displaystyle 10 fulfill the requirements. Therefore, the numerator can be factored into the following:

\displaystyle {x^2+9-10}=(x-1)(x+10)

Because the quantity \displaystyle (x-1) appears in the denominator, this can be "canceled out." This leaves the final answer to be the quantity \displaystyle (x+10).

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