The area of two similar triangles are 12 cm² and 48 cm². If the height of the smaller one is 2.1 cm, then the corresponding height of the bigger one is :

Note : The areas of two similar triangles are in the ratio of the squares of the corresponding altitudes. Let the length of the required altitude be x cm
Then,
$$\eqalign{ & \frac{{12}}{{48}} = \frac{{{{\left( {2.1} \right)}^2}}}{{{x^2}}} \cr & \Rightarrow {x^2} = \left( {4.41 \times 4} \right) \cr & \Rightarrow x = 2.1 \times 2 \cr & \Rightarrow x = 4.2\,cm \cr} $$