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. This means that energy in a closed system cannot be created or destroyed.

2. **Statement of the Law**: The first law of thermodynamics can be stated as:
- "The change in internal energy of a system is equal to the heat added to the system minus the work done by the system."
- Mathematically, it can be expressed as:
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