Answer:
Length = 3 inches
Width = 7 Inches
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