几何的函数表达式题

题目

几何的函数表达式题
在直角梯形ABCD中,∠A=∠D=90度,截取AE=BF=DG=X,已知AB=6,CD=3,AD=4,求四边形CGEF的面积S关于X的函数表达式和X的取值范围.
图:EF有条线连着,因为实在找不到什么横线表示..
D x G C
┌———————————┐
│ /
│ /
│ /
E│ /
│
┗——————————————┚—┚
A F B
`D``x`` G ``````````````C
`┌——————┐
`│```/ `````````````````
`│``/```````````````````
`│`/`````````````````````
E│/```````````````````````
`│ ``````````````````````````
`┗—————————┚—┚
A`````````````````````````````F```B

答案

S=18-2x-0.5x(6-x)-0.5x(4-x)