The angle whose cosine is 8/13 is found using the inverse cosine function:
y° = arccos(8/13) ≈ 52.0°
y ≈ 52.0
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The calculator button to compute this value is probably labeled cos⁻¹. You may need to access the function using a <em>Shift</em> or <em>2nd</em> key. The calculator must be set to degrees mode to prevent the answer from appearing in radians or grads. If you use a spreadsheet, your formula may look like ...
