An economy operates on its production possibility frontier (PPF) as shown in the diagram below.

The opportunity cost of increasing the production of capital goods from Y1 Y_1\,Y1 to Y2 Y_2\,Y2 is represented by:
Y2−Y1Y_2 - Y_1Y2−Y1
X1−X2X_1 - X_2X1−X2
X2−X1X_2 - X_1X2−X1
X2−X1Y2−Y1\frac{X_2 - X_1}{Y_2 - Y_1}Y2−Y1X2−X1