Mixing degrees and radians on a calculator

The sine of 90 is 0.894.

sin(90°) = 1. The value 0.894 is sin(90 rad), because 90 radians correspond to a multiple of 2π. 180° is exactly π radians, about 3.14159265358979.

Calculators and spreadsheets have a DEG/RAD switch that easily sits in the wrong position unnoticed. Because both modes return plausible numbers, the error often only shows up in the result.

Check the mode before any trigonometric function and compute a known value as a control: sin(30°) must give 0.5, cos(0) must give 1. If a result between −1 and 1 looks unexpected, suspect the mode first.