Read the following chapter in the textbook on elasticity. Section 5.1 is the most important.
A note: In section 5.1 the textbook presents a way of calculating elasticity called the “midpoint formula”. It is important to know why the midpoints need to be used in calculating elasticity, but for our purposes in this course I will not be asking you do any calculations using the full, complex “midpoint formula”. The formula looks a lot more complex and scarier than it really is. But nonetheless, calculations you do for elasticity will be simple ones based on
“% change in Qty / % change in price = elasticity”.
So don’t get too tied up in the math. Focus on what the actual measure of elasticity is saying and means.