1. The reciprocal of focal length (f) equals the sum of the reciprocals of the object distance (u) and the image distance (v).   Hint Hint       2. The sum of the parallel sides (a and b) of a trapezium is twice the area (A) divided by the distance (d) between the parallel sides.   Hint       3. Weight (W) is the product of the mass (m) of a body and the acceleration due to gravity (g).   Hint       4. Pressure (P) is the force (F) divided by the area (A).   Hint       5. Volume (V) of a cube equals the cube of the length of an edge (a).   Hint       6. Curved Surface area (S) of a right cone is (22/7) times the product of the radius (r) and the slant height (L).   Hint       7. Distance traveled (s) is the product of speed (u) and time (t).   Hint