A postcard has a perimeter of 20 inches. Its area is 21 square inches. What are the dimensions of the postcard?

Length = 3 inches

Width = 7 Inches

Step-by-step explanation:

The shape of a postcard = Rectangle

L = Length

W = Width

A postcard has a perimeter of 20 inches.

Formula for perimeter of a rectangle = 2L + 2W

Hence:

2L + 2W = 20 Equation 1

Its area is 21 square inches.

The formula for Area of a rectangle = LW

Hence:

LW = 21

L = 21/W

We substitute 21/W for L in Equation 1

2L + 2W = 20 Equation 1

2(21/W) + 2W = 20

42/W + 2W = 20

Multiply both sides by W

42 + 2W² = 20W

2W² - 20W + 42 = 0

2W - 14W - 6W + 42 = 0

(2W - 6)( W - 7) = 0

2W - 6 = 0

2W = 6

W = 6/2 = 3

W - 7 = 0

W = 7 inches

Solving for L

L = 21/W

W = 7

L = 21/7

L = 3 inches

The dimensions of the postcard are:

Length = 3 inches

Width = 7 Inches

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step-by-step explanation: