Derivatives of Trigonometric Functions

Derivatives of Trigonometric Functions The trigonometric functions are a final category of functions that are very useful in many applications. Rather than derive the derivatives for cos(x) and sin(x), we will take them axiomatically, and use them to find the derivatives of other trigonometric functions. d sin(x) = cos(x) dx and d cos(x) = − sin(x) dx To remember which derivative contains the negative sign, recall the graphs of the sine and cosine functions. At x = 0, sin(x) is increasing, and cos(x) is positive, so it makes sense that the derivative is a positive cos(x). On the other hand, just after x = 0, cos(x) is decreasing, and sin(x) is positive, so the derivative must be a negative sin(x). Example 1 Find all derivatives of sin(x). Solution Since we know cos(x) is the derivative of sin(x), if we can complete the above task, then we will also have all derivatives of cos(x). d sin(x) = cos(x) dx gives us the first derivative of the sine function. d d2 sin(x) = cos(x) = − sin(x) 2 dx dx gives us the second derivative. Also d3 d sin(x) = − sin(x) = − cos(x) 3 dx dx Finally d4 d sin(x) = − cos(x) = sin(x) 4 dx dx Now we can see that the fourth derivative of sin(x) is sin(x), so we can easily enough find any derivative of the sine function as follows. Suppose we want to find the nth derivative of sine. All we need to do is divide n by 4, and look at the remainder r. If we take the rth derivative of sine, it will be exactly the same as taking the nth derivative, as every four derivatives will simply return us to the original result of the sine function. Applying this principle, we find that the 17th derivative of the sine function is equal to the 1st derivative, so d17 d sin(x) = sin(x) = cos(x) 17 dx dx The derivatives of cos(x) have the same behavior, repeating every cycle of 4. The nth derivative of cosine is the (n + 1)th derivative of sine, as cosine is the first derivative of sine. The derivatives of sine and cosine display this cyclic behavior due to their relationship to the complex exponential ix −ix function. Euler’s formula states that eix = cos(x) + i sin(x), so we find that cos(x) = e +e and 2 eix −e−ix sin(x) = . Taking the derivative of these two equations provides an alternative method to 2i find the derivatives of sine and cosine, as well as elucidating the link between these elusive functions. Knowledge of the derivatives of sine and cosine allows us to find the derivatives of all other trigonometric functions using the quotient rule. Recall the following identities: tan(x) = sin(x) cos(x) cot(x) = cos(x) sin(x) sec(x) = 1 cos(x) csc(x) = 1 sin(x) Example 2 Find the derivative of csc(x). 1 Solution First recall that csc(x) = sin(x) . Now let us apply the quotient rule sin(x) · 0 − 1 · cos(x) cos(x) 1 d csc(x) = =− · = − cot(x) csc(x) 2 dx sin(x) sin(x) sin(x) Example 3 Find the derivative of ex · cos(x) + x2 . Solution To find this derivative, we will utilitize the sum and product rules. d x (e · cos(x) + x2 ) = ex · cos(x) + ex · (− sin(x)) + 2x = ex (cos(x) − sin(x)) + 2x dx Example 4 Find the derivative of a general sinusoidal function. Solution Recall that the form of a general sinusoidal function is f (t) = A + B · cos
2π (t − φ) T Thus, in order to find the derivative we will need to apply the chain rule.

2π
d 2π 2π 2Bπ 2π (A + B · cos (t − φ) ) = B(− sin (t − φ) ) · =− sin (t − φ) dt T T T T T We can see in the above equation that the smaller the period T becomes, the larger the derivative is. This makes sense as the frequency is inverse of the period f = T1 , so the derivative above confirms our intuition that waveforms with high frequency change rapidly. Example 5 The blood flow in a whale’s aerties is pulsatile. Suppose the blood flow along the artery of a whale is 2πt F (t) = 212 cos( ) 10 where F is measured in liters per second and t is measured in seconds. a. Find the average flow and amplitude. b. Find the period of this flow. How many heartbeats does this whale have per minute? c. When is the flow zero? What is happening at these times? d. Find the rate of change of the flow. What does it mean when this is zero? What is the flow at these times? Solution a. The average is 0 and the amplitude is 212 l/sec. b. The period is 10 seconds. This corresponds to a heartbeat every 10 seconds, which means the heart beats 6 times per minute. nπ 5n c. The flow is zero when 2πt 10 = 2 , where n is an odd integer. Thus, it is zero when t = 2 = 2.5n. At these times the blood has stopped flowing, and is changing direction of flow to the opposite direction. 2πt 2πt d. The rate of change of the flow is F 0 (t) = −212 2π 10 sin( 10 ) = −41.4π sin( 10 ). The derivative is 2πt at a zero when 10 = nπ, where n is an integer. Thus, it is zero when t = 5n. When F 0 (t) = 0, it means that the flow is at a maximum in one direction or the other. When t = 5n, n even, F (t) = 212 l/sec. When t = 5n, n odd, F (t) = −212 l/sec. Example 6 At Barrow, Alaska, atmospheric CO2 levels in parts per million can be modeled by C(x) = 7.5 sin(2πx) + .04x2 + .6x + 330 where x is in years and x = 0 is the year 1960. Find C 0 (x) for the year 2003. What does this mean? Solution First let us find C 0 (x). C 0 (x) = 15π cos(2πx) + 0.08x + 0.6 For the year 2003, we have x = 43. It follows that C 0 (43) = 15π cos(86π) + 0.08 · 43 + 0.6 = 15 + 34.4 + 0.6 = 50 This result tells us that in the year 2003, the atmospheric CO2 levels are increasing by 50ppm. This is compared to a rate of 15.6 ppm in the year 1960.