8 Mohan wants to buy a trapezium-shaped field. Its side along the river is parallel to and twice the side along the road. If the area of this field is $10500 \mathrm{~m}^2$ and the perpendicular distance between the two parallel sides is $100$ m, find the length of the side along the river.
Let the length of the side along the road be $x$ m.
Then according to the question, the length of the side along the river will be $2x$ m.
Area of trapezium $=\frac{1}{2} h(a+b)$
So the equation becomes :
$\frac{1}{2} 100(x+2 x)=10500$
$3x=210$
$x=70$
So the length of the side along the river is $2x=140m$.