I have answered many questions asked by teachers and professors, and very few, if any, deigned to rate the posts. Why do I set myself up for yet another disappointment: I just like to help.
Anyway, the most common source of errors with computation of trigonometric functions is the angle unit. On Ti 83/84Plus (SE) calculators there are two angle units: the degree and the radian.
To verify which angle unit is set as default on a calculator, press MODE. You will see the following screen. The relevant line is the 3rd. On the screen RADIAN is highlighted, meaning that all angle values you feed to trigonometric functions are
interpreted as radian measures.
Similarly, all values returned by inverse
trigonometric functions are in radians.
I do not know what grades you teach, but if you are doing any differential calculus, the only appropriate unit is the radian. Only for the radian unit is the derivative of sin(x) equal to -cos(x). For the degree, you must introduce a factor to adjust (as a change of variable).
Set all your calculators to use the angle unit you are using in your teaching, and there will be no unexpected results.
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