Question

A rectangle has a length 7 meters more than seven times the

width. Find all of the possible widths that result in the area of
the rectangle is less than 392 meters².

Answer 1

The width, x, is in the interval (0, 7), or 0 < x < 7.

Explanation:

width = x

length = 7x + 7

area = x(7x + 7)

x(7x + 7) < 392

7x² + 7x - 392 < 0

x² + x - 56 < 0

(x - 7)(x + 8) < 0

We have two points of interest on the number line:

-8 and 7

That makes three intervals of interest:

(-∞, -8), (-8, 7), (7, ∞)

Now we test a number from each interval.

Try -10:

(x - 7)(x + 8) < 0

(-10 - 7)(-10 + 8) < 0

(-17)(-2) < 0

34 < 0 False

Interval (-∞, -8) does not work.

Try 0:

(x - 7)(x + 8) < 0

(0 - 7)(0 + 8) < 0

(-7)(8) < 0

-56 < 0 True

Interval (-8, 7) works.

Try 10:

(x - 7)(x + 8) < 0

(10 - 7)(10 + 8) < 0

(3)(18) < 0

54 < 0 False

Interval (7, ∞) does not work.

Since we are dealing with a rectangle, the length and width must be positive numbers.

The interval (-8, 7) works mathematically for the width, but it must be limited to positive numbers, so the width must be in the interval

(0, 7)