Question about Texas Instruments TI-83 Plus Calculator

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Radian mode does not work properly

I keep getting wrong answers with the radians mode on. What i typed in was cos(20π + (π/2)) and I get a weird answer (-1E-13) when i know the answer should be zero because i did it on another calculater and that's what i got. Please help me, i have no idea what could be wrong because with other specific things (sin( 20π + (π/2)) for example) ive typed in it does give the right answer, but i don't know why with the other one it doesn't.
Thank you,

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Re: radian mode does not work properly

-1E-13 is a very small number. When doing this kind of a problem you can regard 1E-13 as 0.

Remember that pi is an irrational number. It is only estimated on your calculator. I just played around with a TI-83 and found the following answers:

cos(pi/2) = 0
cos(2*pi + pi/2) = 0
cos(4*pi + pi/2) = 1E-13
cos(20*pi + pi/2) = -1E-13

As you know, that correct answer to each of these is 0. The calculator gives non-zero answers because some very small errors are accumulating. There is nothing wrong with your calculator.

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