State the first law of thermodynamics.

Explanation

### Step-by-Step Solution:

1. **Understanding the Concept**: The first law of thermodynamics is fundamentally based on the principle of conservation of energy. It states that energy cannot be created or destroyed; it can only be transformed from one form to another.

2. **Definition**: The first law of thermodynamics can be formally stated as:
\[ \Delta U = Q - W \]
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