Question 1205097
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I'm not 100% certain if this is the case with Casio calculators, but check out this page here
<a href="https://stackoverflow.com/questions/345085/how-do-trigonometric-functions-work">https://stackoverflow.com/questions/345085/how-do-trigonometric-functions-work</a>


The CORDIC algorithm can be used to compute cos(pi/2) and sin(pi/2)
Since pi/2 is a common angle, likely your calculator will retrieve the following from a lookup table.
cos(pi/2) = 0 
sin(pi/2) = 1


Then under the hood your calculator would then use the identity that
cot = cos/sin
So you'll have 0 up top and 1 down below. 
The ultimate result is 0. This is the correct value of cot(pi/2)


Why does your calculator throw an error when computing 1/tan?
Because tan = sin/cos, and cos(pi/2) = 0.
So tan(pi/2) = 1/0 = undefined
tan(pi/2) being undefined leads to the 1/tan also undefined (even though it should be 0).


Technically what's going on is that as x gets closer to pi/2 from the left, tan(x) approaches positive infinity. 
Stuff of the form 1/tan will approach 0 because you have something like 1/(some really large number). 
If you haven't covered limits yet then don't worry. 
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