Решение:
$$sin(x) + cos(x) = \frac{\sqrt{2}}{2} sin(x)$$
$$cos(x) = \frac{\sqrt{2}}{2} sin(x) - sin(x)$$
$$cos(x) = sin(x)(\frac{\sqrt{2}}{2} - 1)$$
$$cos(x) = sin(x)(\frac{\sqrt{2} - 2}{2})$$
$$\frac{cos(x)}{sin(x)} = \frac{\sqrt{2} - 2}{2}$$
$$ctg(x) = \frac{\sqrt{2} - 2}{2}$$
$$x = arcctg(\frac{\sqrt{2} - 2}{2}) + \pi n, n \in Z$$
Ответ: x = $$arcctg(\frac{\sqrt{2}-2}{2})$$ + $$\pi n$$, n∈Z