The geometry questions on the GRE will require you to draw from basic geometry knowledge, only, but you'll need to be familiar with all of the basics and you'll need to know how to employ different concepts to solve a single problem. For example:
If a triangle with a base of 10ft
has the same area as a circle with radius 3ft,
then what is the height of the triangle?
A triangle, a circle? Areas? Huh? Is there a formula? How do they relate? They don't, but what you'll need to figure out is that you are merely dealing with the area formula for a triangle and a circle, both of which are easy formulas. A circle with radius of 3ft has an area of pi*r2, and if r=3, then the area of the circle is 9pi. So 9pi is equal to the area of a triangle, and we know that the area formula for a triangle is base times height, divided by 2 (and we know that the base is 10). So we set up the following equation, and follow through with the calculations:
9pi = (10 * height) / 2
18pi = 10 * height
18pi/10 = height
Don't be fooled! The geometry questions are generally straight forward, and if you think you need to use a complicated formula to get the answer, then think harder, because most likely there is a shortcut. As a rule of thumb, write down what you know and what is given, and from that information, you should be able to arrive at the answer in at most 3 or 4 steps.