(1)

\begin{align*} \alpha & =\arcsin\frac{b}{\sqrt{a^{2}+b^{2}}}\\ & =\arccos\frac{a}{\sqrt{a^{2}+b^{2}}} \end{align*} とおくと、
\begin{align*} a\sin\theta+b\cos\theta & =\sqrt{a^{2}+b^{2}}\left(\sin\theta\frac{a}{\sqrt{a^{2}+b^{2}}}+\cos\theta\frac{b}{\sqrt{a^{2}+b^{2}}}\right)\\ & =\sqrt{a^{2}+b^{2}}\left(\sin\theta\cos\alpha+\cos\theta\sin\alpha\right)\\ & =\sqrt{a^{2}+b^{2}}\sin(\theta+\alpha) \end{align*}

(2)

(1)より、
\begin{align*} a\sin\theta+b\cos\theta & =\sqrt{a^{2}+b^{2}}\sin(\theta+\alpha)\\ & =\sqrt{a^{2}+b^{2}}\cos(\theta+\alpha-\frac{\pi}{2})\\ & =\sqrt{a^{2}+b^{2}}\cos(\theta-\beta)\qquad,\qquad\beta=\frac{\pi}{2}-\alpha \end{align*} \begin{align*} \beta & =\frac{\pi}{2}-\alpha\\ & =\frac{\pi}{2}-\arcsin\frac{b}{\sqrt{a^{2}+b^{2}}}\\ & =\arccos\frac{b}{\sqrt{a^{2}+b^{2}}} \end{align*} 同様に、
\begin{align*} \beta & =\frac{\pi}{2}-\alpha\\ & =\frac{\pi}{2}-\arccos\frac{a}{\sqrt{a^{2}+b^{2}}}\\ & =\arcsin\frac{a}{\sqrt{a^{2}+b^{2}}} \end{align*} これより、
\begin{align*} \beta & =\arccos\frac{b}{\sqrt{a^{2}+b^{2}}}\\ & =\arcsin\frac{a}{\sqrt{a^{2}+b^{2}}} \end{align*}

(2)別解

\begin{align*} \beta & =\arccos\frac{b}{\sqrt{a^{2}+b^{2}}}\\ & =\arcsin\frac{a}{\sqrt{a^{2}+b^{2}}} \end{align*} とおくと、
\begin{align*} a\sin\theta+b\cos\theta & =\sqrt{a^{2}+b^{2}}\left(\cos\theta\frac{b}{\sqrt{a^{2}+b^{2}}}+\sin\theta\frac{a}{\sqrt{a^{2}+b^{2}}}\right)\\ & =\sqrt{a^{2}+b^{2}}\left(\cos\theta\cos\beta+\sin\theta\sin\beta\right)\\ & =\sqrt{a^{2}+b^{2}}\sin(\theta-\beta) \end{align*}