Explain the law of variable proportion with the help of diagram/ schedule.

Or

What is the likely behavior of total product/marginal product when only one input is increased for increasing production? Used diagram/Schedule.

This law states that
In case of TP:
"If more and more units of a variable factor are employed with fixed factors, total product (TP) increases at diminishing rate and finally starts falling."
In case of MP:
"If quantities of a certain variable factor are increased while quantities of other factors are fixed, MP first increases, then may become constant and finally declines."
Tabular and Diagrammatic presentation:

Behaviour of TPP and MPP:
First Phase: TPP (TP) increases with increasing rate upto A point. MPP (MP) also increases and becomes maximum at point C.
Second Phase: TPP increases with diminishing rate and it is maximum at point B. MPP starts to decline and becomes zero at D point.
Third phase: TPP starts to decline and MPP becomes negative.

Assumptions of the law: The law operates on the basis of the following assumptions:

1. Technique of production does not change.
2. Different quantities of one factor can be varied with fixed factors.
3. All units of variable factors are equally important.
4. Factors of production are not perfect substitutes.
5. There is short period of operation.