The length of the rectangle is 10 cm, and the width is a cm. Write it down as an expression of its perimeter and the semi-perimeter.

A rectangle is a rectangle whose opposite sides are equal and parallel.
Perimeter is the sum of all sides of the figure. Hence the formula: P = a + b + c + d, but since the opposite sides are equal, the formula takes the following form: P = 2a + 2b
Substitute the known values into the formula and get: P = 2a + 20

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