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Differentiate $y=27 \sec^3(x)$ with respect to $x$.

I tried splitting the $\sec^3(x)$ into $\sec^2(x)\cdot \sec(x)$ and using the product rule but that didn't work.

Yadati Kiran
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madeye moody
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3 Answers3

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$$y=27 \sec^3(x)$$ $$y=27 \times(\sec (x))^3$$

Now apply the chain rule...

The "outside" function is the function 'to cube'

The "inside" function is the $\sec(x)$

So focusing on the cube on the "outside"...

$$y=27 \times (inside)^3$$ $$\frac{dy}{dx}=27\times 3 \times (inside)^2 \times (derivative-of- inside)$$

then focussing on the "inside"... $$\frac{dy}{dx}=27\times 3 \times (\sec(x))^2 \times (derivative-of- \sec(x))$$

$$\frac{dy}{dx}=27\times 3 \times (\sec(x))^2 \times (\sec(x)\tan(x))$$

$$\frac{dy}{dx}=81\times (\sec(x))^3 \times \tan(x)$$

$$\frac{dy}{dx}=81 \sec^3(x) \tan(x)$$

Martin Hansen
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  • Okay, so then it'd just be simple differentiation, by multiplying the co-efficient by 3, reducing the power by one and then multiplying by the differential of the function which would be 1? – madeye moody Mar 15 '19 at 21:10
  • Hope that makes sense, it's looks more complicated when it's written out than when you do it - This is the way most schools teach students to think about it rather than the formal substitution method that gets written in a text book - but go with whatever works for you – Martin Hansen Mar 15 '19 at 21:26
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If $y = 27\sec^{3}(x)$, then you can find $y'$ in a couple of ways.

Using the method you suggested:

$$y = 27\sec^{3}(x) = 27\sec^{2}(x)\cdot\sec(x)$$ Applying the product rule, we have: $$y' = 27\dfrac{d}{dx}\big[\sec^{2}(x)\cdot\sec(x) \big] = 27\bigg(\dfrac{d}{dx}\big[\sec^{2}(x)\big]\cdot \sec(x) + \sec^{2}(x)\dfrac{d}{dx}\big[\sec(x)\big]\bigg)$$ One would need to find $\dfrac{d}{dx}\sec^{2}(x)$ by the chain rule (or by applying the product rule again).

By the chain rule: $$y = (f\circ g)(x) = f(g(x)),\quad \text{ where } g(x) = \sec(x),\quad f(x) = 27x^3$$ Then $$y' = f'(g(x))\cdot g'(x) = 3\cdot (\sec(x))^2 \cdot g'(x)$$

Does this make sense?

Thomas Davis
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  • Yes, but I think I'll go with the substitution method someone else provided. Seems simpler and thank you for the effort you put into answering. – madeye moody Mar 15 '19 at 21:12
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Rewrite $y=z^3$ where $z=3\sec x$. As $z$ is a function of $x$ $$\dfrac{dy}{dx}=3z^2\cdot \dfrac{dz}{dx}\;\text{with}\;\dfrac{dz}{dx}=3\sec x\tan x$$ So $$\dfrac{dy}{dx}=27\sec^2 x\cdot(3 \sec x\tan x)$$

Yadati Kiran
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