I am referring to, specifically the sin.z=z and cos.z=1. I am reading Euler's Introductio In Analysin Infinitorium, Vol. I, Ch. VIII and he jumps from binomials to assuming the arc z is infinitely small. I'm not sure why he does this. Could someone help explain what he means by this part of his text and why the approximate quantities, sin.z=z and cos.z=1, are true?
Here is a screenshot of the section of text I am referring to.
