1. Area (A) of a square equals the square of the length of a side (a).   Hint Hint       2. The reciprocal of focal length (f) equals the sum of the reciprocals of the object distance (u) and the image distance (v).   Hint       3. Density (D) is the mass (M) of a body divided by its volume (V).   Hint       4. Weight (W) is the product of the mass (m) of a body and the acceleration due to gravity (g).   Hint       5. Circumference (C) of a circle is (22/7) times the diameter (d).   Hint       6. Area (A) of a rectangle is the product of its length (L) and breadth (B).   Hint       7. Hundred times the simple interest (I) equals the product of the principal (P), rate per cent per annum (r) and time in years (t).   Hint