The length and breadth of some rectangles are given. Find their perimeter using the formula.
1) \[l = 3.5{\text{ m}}\], \[b = 5{\text{ m}}\]
2) \[l = 4.2{\text{ m}}\], \[b = 15{\text{ m}}\]

Hint: The perimeter of the rectangle is calculated using the formula, \[P = 2\left( {l + b} \right)\], where \[l\] is the length of the rectangle and \[b\] is the breadth of the rectangle.
Apply this property, and then use the given conditions to find the required value.

Complete step by step answer
1). Given that the length of the rectangle is \[3.5\] m and the breadth of the rectangle is 5 m.

We know that the perimeter of the rectangle is calculated using the formula, \[P = 2\left( {l + b} \right)\], where \[l\] is the length of the rectangle and \[b\] is the breadth of the rectangle.

We will now find the perimeter of the rectangle using the above formula.

\[P = 2\left( {l + b} \right)\]

Substituting the values of \[l\] and \[b\] in the above expression, we get

\[
   \Rightarrow P = 2\left( {3.5 + 5} \right) \\
   \Rightarrow P = 2\left( {8.5} \right) \\
   \Rightarrow P = 17{\text{ m}} \\
 \]

Thus, the perimeter of this rectangle is 17 meters.

2). Given that the length of the rectangle is \[4.2\] m and the breadth of the rectangle is 15 m.

We know that the perimeter of the rectangle is calculated using the formula, \[P = 2\left( {l + b} \right)\], where \[l\] is the length of the rectangle and \[b\] is the breadth of the rectangle.

We will now find the perimeter of the rectangle using the above formula.

\[P = 2\left( {l + b} \right)\]

Substituting the values of \[l\] and \[b\] in the above expression, we get

\[
   \Rightarrow P = 2\left( {4.2 + 15} \right) \\
   \Rightarrow P = 2\left( {19.2} \right) \\
   \Rightarrow P = 38.4{\text{ m}} \\
 \]

Thus, the perimeter of this rectangle is \[38.4\] meters.


Note: In solving these types of questions, you should be familiar with the formula of perimeter of the rectangle. The key concept to solve such types of questions, we have to remember is that in the rectangle opposite sides are equal and then calculate the perimeter of that rectangle by adding all the sides.