Cho biết tính: sin15° - √(cos12°) - sin90°

To calculate \( \sin 15^\circ - \sqrt{\cos 12^\circ} - \sin 90^\circ \), we'll evaluate each term one by one:

1. **Calculate \( \sin 15^\circ \)**:
\[
\sin 15^\circ = \sin(45^\circ - 30^\circ) = \sin 45^\circ \cos 30^\circ - \cos 45^\circ \sin 30^\circ = \frac{\sqrt{2}}{2} \cdot \frac{\sqrt{3}}{2} - \frac{\sqrt{2}}{2} \cdot \frac{1}{2}
\]
\[
= \frac{\sqrt{6}}{4} - \frac{\sqrt{2}}{4} = \frac{\sqrt{6} - \sqrt{2}}{4}
\]

2. **Calculate \( \sqrt{\cos 12^\circ} \)**:
- We can use the calculator or trigonometric tables to find \( \cos 12^\circ \) (approximately \( 0.978 \)), then:
\[
\sqrt{\cos 12^\circ} \approx \sqrt{0.978} \approx 0.989
\]

3. **Calculate \( \sin 90^\circ \)**:
\[
\sin 90^\circ = 1
\]

Now we combine these results:
\[
\sin 15^\circ - \sqrt{\cos 12^\circ} - \sin 90^\circ \approx \left( \frac{\sqrt{6} - \sqrt{2}}{4} \right) - 0.989 - 1
\]

Performing this calculation:
1. \( \frac{\sqrt{6} - \sqrt{2}}{4} \)
2. \( - 0.989 - 1 = -1.989 \)

Finally, compute the total:
\[
\frac{\sqrt{6} - \sqrt{2}}{4} - 1.989
\]

Since \( \sqrt{6} \approx 2.45 \) and \( \sqrt{2} \approx 1.41 \):
\[
\frac{2.45 - 1.41}{4} - 1.989 \approx \frac{1.04}{4} - 1.989 \approx 0.26 - 1.989 \approx -1.729
\]

Thus, the value of \( \sin 15^\circ - \sqrt{\cos 12^\circ} - \sin 90^\circ \) is approximately **-1.729**.