How to Compute the Derivative of a Trigonometric Function
Trigonometric functions, also called circular functions, are functions of angles. You’re probably already familiar with the six trigonometric functions: sin x, cos x, tan x, sec x, csc x and cot x. There’s a table that can help you figure out derivatives for the six trig functions:
d/dx sin x = cos x
d/dx csc x = -csc x cot x
d/dx cos x = – sin x
d/dx sec x = sec x tan x
d/dx tan x = sec2 x
d/dx cot x = – csc2 x
However, you might be asked (especially in beginning calculus) to find the derivative of a trig function using the definition of a derivative instead of a table. When you use the definition of a derivative, you’re actually working on a proof. In other words, if you want to prove that one function is a derivative of another, you’ll nearly always start with the definition of a derivative and end with the derivative of the trigonometric function.
Sample Problem: Find the derivative of a trigonometric function (sin x) using the definition of a derivative (in other words, prove that d/dx sin x = cos x:
Step 1: Insert the function sin x into the definition of a derivative:
Step 2: Use the trigonometric identity sin(a+b)=sin a * cos B + cos a * sin B to rewrite the definition from Step 1:
Step 3: Use algebra to rewrite the formula in Step 2:
= – sin x* (0) + cos x * (1) = cos x
Tip: You can use the exact same technique to work out a proof for any trigonometric function. Start with the definition of a derivative and identify the trig functions that fit the bill.