7.4. Find the number of all integer solutions of the inequality $\sqrt{3 \cos \frac{\pi x}{2}+\cos \frac{\pi x}{4}+1}+\sqrt{6} \cdot \cos \frac{\pi x}{4} \geq 0$, belonging to the interval $[1991 ; 2013]$.
Answer: 9. Comments. Solution of the inequality: $-\frac{4}{3}+8 k \leq x \leq \frac{4}{3}+8 k$. From the given interval, the following will be included in the answer: 1991, 1992, 1993, 1999, 2000, 2001, 2007, 2008, 2009 - 9 numbers.