This partial production function shows how output increases as more capital is being employed while work-hours remain fixed. Its graph is obtained by placing a vertical cut through the 3D production function parallel to the axis measuring the capital stock.

The slope of the partial production function measures how much output is gained by a given small increase of the capital stock. This slope is called the marginal product of capital. The two tangency lines measure this marginal product of capital at K₁ and K₂. They indicate that the marginal product of capital becomes smaller and smaller as the capital stock rises.

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