(2)4 square meters and 9 square decimeters =( 4) square meters =( 90) square decimeters.
(3) After the base and height of the parallelogram are tripled, its area is expanded to (9) times of the original.
(4) The length of an equilateral triangle is one meter and the circumference is (3a) meters.
(5) The channel section is trapezoidal, with the mouth width 1.8m, the bottom width 1.4m and the depth of 0.9m, and the channel section is (1.44) m2.
( 1.8+ 1.4)x 0.9÷2 = 1.6x 0.9 = 1.44