Solve for X: Rectangle Area x²-13 with Dimensions (x-4) and (x+1)

Question

Observe the rectangle below.

x>0

If the area of the rectangle is:

x213 x^2-13 .

Calculate x.

x-4x-4x-4x+1x+1x+1x²-13

Video Solution

Solution Steps

00:00 Find X
00:03 Use the formula for calculating rectangle area (side times side)
00:10 Substitute appropriate values according to the given data and solve for X
00:18 Expand brackets properly - each term multiplies each term
00:36 Simplify what we can
00:42 Isolate X
00:59 And this is the solution to the question

Step-by-Step Solution

First, recall the formula for calculating the area of a rectangle:

The area of a rectangle (which has two pairs of equal opposite sides and all angles are 90° 90\degree ) with sides of length a,b a,\hspace{4pt} b units, is given by the formula:

S=ab \boxed{ S_{\textcolor{blue}{\boxed{\hspace{6pt}}}}=a\cdot b } (square units)

90°90°90°bbbaaabbbaaa

After recalling the formula for the area of a rectangle, let's proceed to solve the problem:

Begin by denoting the area of the given rectangle as: S S_{\textcolor{blue}{\boxed{\hspace{6pt}}}} and proceed to write (in mathematical notation) the given information:

S=x213 S_{\textcolor{blue}{\boxed{\hspace{6pt}}}}=x^2-13

Continue to calculate the area of the rectangle given in the problem:

x-4x-4x-4x+1x+1x+1x²-13

Using the rectangle area formula mentioned earlier:

S=abS=(x4)(x+1) S_{\textcolor{blue}{\boxed{\hspace{6pt}}}}=a\cdot b\\ \downarrow\\ S_{\textcolor{blue}{\boxed{\hspace{6pt}}}}=(x-4)(x+1)

Continue to simplify the expression that we obtained for the rectangle's area, using the distributive property:

(c+d)(h+g)=ch+cg+dh+dg (c+d)(h+g)=ch+cg+dh+dg

We are able to obtain the area of the rectangle by

using the distributive property as shown below:

S=(x4)(x+1)S=x2+x4x4S=x23x4 S_{\textcolor{blue}{\boxed{\hspace{6pt}}}}=(x-4)(x+1) \\ S_{\textcolor{blue}{\boxed{\hspace{6pt}}}}=x^2+x-4x-4\\ \boxed{ S_{\textcolor{blue}{\boxed{\hspace{6pt}}}}=x^2-3x-4}

Recall the given information:

S=x213 S_{\textcolor{blue}{\boxed{\hspace{6pt}}}}=x^2-13

Therefore, we can conclude that:

x23x4=x2133x=4133x=9/(3)x=3 x^2-3x-4=x^2-13 \\ \downarrow\\ -3x=4-13\\ -3x=-9\hspace{6pt}\text{/}(-3)\\ \boxed{x=3}

We solved the resulting equation simply by combining like terms, isolating the expression with the unknown on one side and dividing both sides by the unknown's coefficient in the final step,

Note that this result satisfies the domain of definition for x, which was given as:

-1\text{<}x\text{<}4 and therefore it is the correct result

The correct answer is answer C.

Answer

x=3 x=3