\(y=sinx+cosx=\sqrt{2}sin\left(x+\dfrac{\pi}{4}\right)\)
\(sin\left(x+\dfrac{\pi}{4}\right)\in\left[-1;1\right]\Rightarrow y=\sqrt{2}sin\left(x+\dfrac{\pi}{4}\right)\in\left[-\sqrt{2};\sqrt{2}\right]\)
\(\Rightarrow y_{max}=\sqrt{2},y_{min}=-\sqrt{2}\)