Trigonometry Examples

$\cos (\arcsin (\frac{2}{5}))$

Step 1

Draw a triangle in the plane with vertices $(\sqrt{1^{2} - {(\frac{2}{5})}^{2}}, \frac{2}{5})$ , $(\sqrt{1^{2} - {(\frac{2}{5})}^{2}}, 0)$ , and the origin. Then $\arcsin (\frac{2}{5})$ is the angle between the positive x-axis and the ray beginning at the origin and passing through $(\sqrt{1^{2} - {(\frac{2}{5})}^{2}}, \frac{2}{5})$ . Therefore, $\cos (\arcsin (\frac{2}{5}))$ is $\sqrt{\frac{21}{25}}$ .

$\sqrt{\frac{21}{25}}$

Step 2

Rewrite $\sqrt{\frac{21}{25}}$ as $\frac{\sqrt{21}}{\sqrt{25}}$ .

$\frac{\sqrt{21}}{\sqrt{25}}$

Step 3

Simplify the denominator.

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Step 3.1

Rewrite $25$ as $5^{2}$ .

$\frac{\sqrt{21}}{\sqrt{5^{2}}}$

Step 3.2

Pull terms out from under the radical, assuming positive real numbers.

$\frac{\sqrt{21}}{5}$

Step 4

The result can be shown in multiple forms.

Exact Form:

$\frac{\sqrt{21}}{5}$

Decimal Form:

$0.91651513 \dots$