Assume there are only two goods produced in the economy (having fixed amount of resources and technology), Good A and Good B. The quantities of Good A and Good B which can be produced using given resources are plotted on a graph with good A on x-axis and Good B on y-axis. Different combinations of A and B when plotted and joined gave a downward sloping concave curve. One point, Point F lies above the curve. Point E lies below the curve.

Why is the obtained curve concave to origin?

Increasing marginal rate of transformation