Chain Quotient Order

Calculus Level 1

Find f ( 0 ) f'(0) if f ( x ) = e sin ( x ) sec 2 ( x ) \large f(x) = \frac {e^{\sin(x)}}{\sec^2 (x)} .


The answer is 1.00.

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2 solutions

Jake Lai
Apr 3, 2015

It's actually a product rule question in disguise.

We can split f ( x ) = e sin x sec 2 x = e sin x cos 2 x f(x) = \frac{e^{\sin x}}{\sec^{2} x} = e^{\sin x}\cos^{2} x into g ( x ) = e sin x g(x) = e^{\sin x} and h ( x ) = cos 2 x h(x) = \cos^{2} x .

Then, f ( x ) = g ( x ) h ( x ) + g ( x ) h ( x ) f'(x) = g(x)h'(x) + g'(x)h(x) by the product rule. This means we have to find g ( x ) g'(x) and h ( x ) h'(x) first. By the chain rule,

g ( x ) = ( e sin x ) = e sin x cos x g'(x) = \left( e^{\sin x} \right)' = e^{\sin x}\cos x

h ( x ) = ( cos 2 x ) = 2 cos x sin x h'(x) = \left( \cos^{2} x \right)' = -2\cos x \sin x

Hence,

f ( x ) = e sin x cos 3 x 2 e sin x cos x sin x f'(x) = e^{\sin x}\cos^{3} x - 2 e^{\sin x}\cos x \sin x

We can then substitute in x = 0 x = 0 to obtain f ( 0 ) = 1 f'(0) = \boxed{1} .

Moderator note:

Yes, the convention method works. One could also use Chain Rule to solve this. f ( x ) = cos 2 x e sin ( x ) = ( 1 sin 2 x ) e sin ( x ) f(x) = \cos^2 x \cdot e^{ \sin (x) } = (1 - \sin^2 x) e^{ \sin(x) } . Can you continue from here?

Manthar Ali
Apr 1, 2015

Applying y=u/v form so here u = e^sinx and v = sec^2x Derivative of u= cosxe^sinx and v = 2secx secxtanx General derivative rule of u/v is (U dv/dx -vdv/dx)/v^2 f'(x)= cosxe^sinx - 2 secx secx tandem e^sinx / sec^4x f'(0)= 1 Because e^0=1, sec0=1, tan0= 0

Moderator note:

Quotient Rule is a specific case for Product Rule. Note that the denominator is sec 2 x = ( cos 2 x ) 1 \sec^2 x = \left (\cos^2 x \right)^{-1} . It's slightly simpler to differentiate cos 2 x \cos^2 x than sec 2 x \sec^2 x don't you think? Furthermore, you don't have to deal with fractions anymore.

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