a sin x + b cos x = $\sqrt {a^2 + b^2} (\frac{a}{\sqrt {a^2 + b^2}} sin x + \frac{b}{\sqrt {a^2 + b^2}}cos x)$
Do $\frac{a}{\sqrt {a^2 + b^2}}^2 + \frac{b}{\sqrt {a^2 + b^2}}^2 = 1$, ta đặt $@ = arccos (\frac{a}{\sqrt {a^2 + b^2}})$
thì a sin x + b cos x = $\sqrt {a^2 + b^2} (sin x cos @ + cos x sin @)$ = $\sqrt {a^2 + b^2} sin (x + @)$ = $\sqrt {a^2 + b^2} sin (x + arccos (\frac{a}{\sqrt {a^2 + b^2}}))$