$\"\"$

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Observe the figure:

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The values of the right triangle $\"\"$ and $\"\"$ .

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$\"\"$ (Defintion of cosine)

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$\"\"$ (Substitute $\"\"$ and $\"\"$ )

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$\"\"$ (Divide: $\"\"$ )

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$\"\"$ (Inverse function)

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$\"\"$ (Use calculator)

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$\"\"$ .

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$\"\"$

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$\"\"$ .