How do you use a calculator to evaluate $tan(-pi/9)$ ?

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Answer 1 · 2016-12-12T14:22:16

Convert $(-pi/9)$ to degrees.
$tan (-pi/9) = tan ((- 180)/9) = tan (- 20^@)$
Calculator gives:
tan (-20) = - 0.3639

How do you use a calculator to evaluate tan(-pi/9)tan(-pi/9)?