Equation of simple harmonic progressive wave is given by $y=\frac{1}{\sqrt{a}}\sin(\omega t)\pm \frac{1}{\sqrt{b}}\cos(\omega t)$. Then the resultant amplitude of the wave is ($\cos 90^{\circ}=0$):
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Logic Tip: Orthogonal wave components (sine and cosine) combine using the Pythagorean theorem for the resultant amplitude.