Find the area of a triangle whose base is 6 cm and corresponding height is 4 cm.

Find the area of a triangle whose sides are 3 cm, 4 cm and 5 cm.

Find the area of a triangle whose sides are 12 cm, 9.6 cm and 7.2 cm.

Find the area of a triangle whose sides are 34 cm, 20 cm and 42 cm. Hence, find the length of the altitude corresponding to the shortest side.