MHB Differentiating a trignometric function

[tmt1]

So, I have

$$\d{x}{t} = 20 sex^2∂ \d{∂}{t}$$

And the text goes to:

$$ \d{∂}{t} = \frac{1}{20}cos^2 ∂ \d{x}{t}$$

I don't understand where the cos comes from? Is it a trigonometric identity? If so I can't find it.

 

[I like Serena]

So, I have

$$\d{x}{t} = 20 sex^2∂ \d{∂}{t}$$

And the text goes to:

$$ \d{∂}{t} = \frac{1}{20}cos^2 ∂ \d{x}{t}$$

I don't understand where the cos comes from? Is it a trigonometric identity? If so I can't find it.

Hi tmt!

By definition:
$$\sec \delta = \frac{1}{\cos\delta}$$