# How do you evaluate using a calculator of tan^-1(-1.7321)?

Aug 19, 2016

tan120° = -1.7321 and tan300° = -1.7321

#### Explanation:

This question is asking "What angle(s) has a tan value of $- 1.7321$

Step 1 . "In which quadrants is tan negative?"

From the CAST rule: $\text{pos in " 1st and 3rd rArr "neg in } 2 n d \mathmr{and} 4 t h$

Step 2 . Find the root angle. Find ${\tan}^{-} 1 \left(+ 1.7321\right)$
This gives the acute angle in the 1st quadrant from which we can get the angles in the other quadrants.

theta = tan^-1(+1.7321) = 60°

Step 3 Find the angles in the 2nd and 4th quadrants:

2nd quadrant: 180°- theta = 180°-60° = 120°

4th quadrant: 360°- theta =360° -60° =300°

CHECK: use a calculator to verify that:

tan120° = -1.7321 and tan300° = -1.7321

These are the angles for 0<= theta<= 360°

Aug 22, 2016

Method shown below.

#### Explanation:

Using my now very outdated Casio fx-9700GE

Shift then tan$\to {\tan}^{-}$

Do not use the 'minus' key but use the 'negative' key. On my machine designated as $\left(-\right)$ showing the brackets.

After using $\left(-\right)$ type in 1.7321

After that I have to use the key marked 'EXE' . Yours may be labelled Ans

Whatever it is it is the one you would have to press make it solve something like 2+3