what is the exact value of cos(7pi/8)

Answer:

cos ([tex]\frac{7\pi}{8}[/tex]) = [tex]-\frac{\sqrt{2+\sqrt{2}}}{2}[/tex]

Decimal value: −0.9238795325...

Step-by-step explanation:

Write,

cos [tex]\frac{7\pi}{8}[/tex] = cos [tex](\pi-\frac{\pi}{8})[/tex] = −cos [tex]\frac{\pi }{8}[/tex]

Now using half angle formula, cos(x) = 2 cos²([tex]\frac{\pi }{2}[/tex]) − 1, we can write cos [tex]\frac{\pi }{4}[/tex] = 2

cos²([tex]\frac{\pi }{8}[/tex]) − 1

This means [tex]\frac{1}{\sqrt{2}}[/tex] + 1 = 2 cos²([tex]\frac{\pi }{8}[/tex])

2 cos²([tex]\frac{\pi }{8}[/tex]) = [tex]\frac{\sqrt{2}+1}{\sqrt{2}}[/tex]

cos²([tex]\frac{\pi }{8}[/tex]) = [tex]\frac{\sqrt{2}+1}{2\sqrt{2}}[/tex]

cos([tex]\frac{\pi }{8}[/tex]) = [tex]\sqrt{\frac{\sqrt{2}+1}{2\sqrt{2}}}[/tex]

Simplify,

[tex]\sqrt{\frac{\sqrt{2}+1}{2\sqrt{2}}}[/tex] = [tex]\sqrt{\frac{2+\sqrt{2}}{4}[/tex] = [tex]\frac{\sqrt{2+\sqrt{2}}}{2}[/tex]

Hence,

cos ([tex]\frac{7\pi}{8}[/tex]) = −cos ([tex]\frac{\pi }{8}[/tex]) = [tex]-\frac{\sqrt{2+\sqrt{2}}}{2}[/tex]

The exact value of cos(7pi/8) is [tex]\rm -\dfrac{\sqrt{2+\sqrt{2}} }{2 }[/tex] and this can be determined by using the trigonometric properties.

Given :

Expression  -- [tex]\rm cos(\dfrac{7\pi}{8})[/tex]

The following steps can be used to determine the exact value of the given expression:

Step 1 - Write the given expression.

[tex]=\rm cos(\dfrac{7\pi}{8})[/tex]

Step 2 - Rewrite the above expression.

[tex]= \rm cos(\dfrac{8\pi - \pi}{8})[/tex]

Step 3 - The above expression is reduced to:

[tex]= \rm -cos(\dfrac{ \pi}{8})[/tex]    --- (1)

Step 4 - According to the trigonometric properties:

[tex]\rm cos(x) = 2 cos^2(\frac{x}{2})-1[/tex]

Step 5 - Substitute the value of x = [tex]\pi[/tex]/4 in the above expression.

[tex]\rm cos(\frac{\pi}{4}) = 2 cos^2(\frac{\pi}{8})-1[/tex]

Step 6 - Simplify the above expression.

[tex]\rm cos(\frac{\pi}{8})=\sqrt{\dfrac{\dfrac{1}{\sqrt{2}} +1}{2}}[/tex]

[tex]\rm cos(\frac{\pi}{8})=\sqrt{\dfrac{1+\sqrt{2} }{2\sqrt{2} }[/tex]

[tex]\rm cos(\frac{\pi}{8})=\dfrac{\sqrt{2+\sqrt{2}} }{2 }[/tex]

Step 7 - Substitute the value of cos([tex]\pi[/tex]/8) in the expression (1).

[tex]\rm -cos(\frac{\pi}{8})=-\dfrac{\sqrt{2+\sqrt{2}} }{2 }[/tex]

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