1. **State the problem:** Given $a+b=10$ and $ab=21$, find $(a-b)^2$.
2. **Recall the formula:** We know that $(a-b)^2 = (a+b)^2 - 4ab$.
3. **Substitute the known values:**
$$
(a-b)^2 = (10)^2 - 4 \times 21
$$
4. **Calculate:**
$$
(a-b)^2 = 100 - 84 = 16
$$
5. **Conclusion:** The value of $(a-b)^2$ is 16.
Difference Square 1Aa4C3
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