Pick the Rectangle Tool(M), create two squares (50 by 50px) and place them as shown in the first image. Fill the top shape with black and the bottom shape with white. Grab the Direct Selection Tool(A) and focus on the black shape. Select the two, bottom anchor points and go to Object > Path > Average. Check Both then click OK. This will turn your square into a triangle as shown in the second image. Move to the bottom square. Again, using the Direct Selection Tool(A), select the top anchor point and go to Object > Path > Average. Check Both then click OK.