the cross - section of a cannel is a trapezium in shape . if the cannel is 12 m wide at the top and 8 m wide at the bottom and the area of cross - section is 690 sq m , the depth of cannel is ?
"1 / 2 * d ( 12 + 8 ) = 690 d = 69 answer : d"
divide(divide(divide(690, divide(add(12, 8), const_2)), 8), const_2)