本人数学底子差，刚开始准备GMAT，所以以后可能会有很多问题讲教各位大虾们：

图我不知怎么传上来，I题目如下：

In the rectangular coordinate system above, the area of (三角形）RST is :

答案是： B（答案是分式形式，我不知如何可以打出来_^）

我不明白怎么就选了B

If segment RT is chosen as the base of △
RST, then the height is b, the
y-coordinate of point S. Since RT = c
– 1 (the difference between the
x-coordinate of R and T), the area of
△RST is 0.5(RT)b = 0.5(c - 1)b.
So, B is the correct answer.

OG解释的很清楚啊，你不明白哪里？