Mathematics for IGCSE & O level - Pythagoras' Theorem And Trigonometry (Section 4 - No. 40)
What do you need to do before using the inverse trigonometric functions (sin⁻¹, cos⁻¹, tan⁻¹) on a calculator?
Switch the calculator to radian mode.
Switch the calculator to degree mode.
Square all of the numbers.
Divide all of the numbers.
Explication
Make sure that your calculator is set to degree mode before using inverse trig functions.