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## Homework Statement

I'm learning now about darivative all by my self (without a teacher) and I'm not sure about this development

## Homework Equations

## The Attempt at a Solution

[tex]Y=senx[/tex]

[tex]y+{\Delta}Y=sen(x+{\Delta}Y)[/tex]

[tex]{\Delta}Y=senx*cos{\Delta}x+ sen{\Delta}x*cosx[/tex]

What about this cos,what is it doing in the equation?

I really don't get it.