5. Найдите значение выражения -k(k + 4) + (k - 6)(6 + k) при k = 1/4.

5. $-k(k + 4) + (k - 6)(6 + k) = -k^2 - 4k + k^2 - 36 = -4k - 36$. При $k = \frac{1}{4}$: $-4 \cdot \frac{1}{4} - 36 = -1 - 36 = -37$. 6. $x^2 + 6x + 9 + (4 - x)(x + 4) = x^2 + 6x + 9 + 16 - x^2 = 6x + 25$. При $x = -\frac{5}{6}$: $6 \cdot (- \frac{5}{6}) + 25 = -5 + 25 = 20$. 7. $(-f - 5)(f - 5) + f(f + 8) = -(f + 5)(f - 5) + f^2 + 8f = -(f^2 - 25) + f^2 + 8f = -f^2 + 25 + f^2 + 8f = 8f + 25$. При $f = \frac{3}{8}$: $8 \cdot \frac{3}{8} + 25 = 3 + 25 = 28$. 8. $n(3 - n) - (7 - n)(n + 7) = 3n - n^2 - (49 - n^2) = 3n - n^2 - 49 + n^2 = 3n - 49$. При $n = -\frac{4}{3}$: $3 \cdot (- \frac{4}{3}) - 49 = -4 - 49 = -53$. 9. $-u(u + 7) + (u + 6)^2 = -u^2 - 7u + u^2 + 12u + 36 = 5u + 36$. При $u = \frac{4}{5}$: $5 \cdot \frac{4}{5} + 36 = 4 + 36 = 40$. 10. $(u - 3)^2 + 6(-1 + u) = u^2 - 6u + 9 - 6 + 6u = u^2 + 3$. При $u = 0.5$: $0.5^2 + 3 = 0.25 + 3 = 3.25$.