Cos x = 3/4 untuk 0°<90° nilai dari sin 3x+sin x=

[tex]
\text{Karena }\cos{x}=\frac{3}{4}\text{, maka}\\
\sin{x}=\sqrt{1-\cos^2{x}}=\sqrt{1-\frac{9}{16}}=\sqrt{\frac{7}{16}}=\frac{\sqrt{7}}{4}\\
\text{Selanjutnya, diperoleh}\\
\sin{3x}+\sin{x}\\
=2\sin{\left(\frac{3x+x}{2}\right)}\cos{\left(\frac{3x-x}{2}\right)}\\
=2\sin{2x}\cos{x}\\
=2\cdot2\sin{x}\cos{x}\cos{x}\\
=4\cdot\frac{\sqrt{7}}{4}\cdot\frac{3}{4}\\
=\frac{3}{4}\sqrt{7}\\
[/tex]