中３數......................

要證明他是梯形，先要證明一組對邊是平行。另外兩邊不是平行
slope of BC=(3-1)/(1-2)=-2
slope of AC=(1-0)/(2-0)=1/2
slope of AD=(4-0)/(-2-0)=-2
slope of BD=(4-1)/(-2-2)=3/-4
Thus, slope of BC=slope of AD
slope of AC is not equal to BD
ABCD is a 梯形
slope of line=(y2-y1)/(x2-x1)

梯形的定義

一個四邊形其中一組對邊平行

所以你要証明

slope of AD = (4-0)/(-2-0) = -2
slope of BC = (3-1)/(1-2) = -2

仲未夠
仲要証埋slope of AB =/= slope of CD
(如果等於的話就會變左平行四邊形)