Merge pull request #23 from guangmingyao/yao_dev

Yao dev
Added a new image to demonstrate the slope and its relationship to the shape of the curve.
Added a new plot demonstrating the role of the slope and the shape of the function's graph
Merge pull request #44 from KellyBlack/black_dev

added new images to demonstrate the slope
made minor changes on lecture 3
Merge pull request #24 from guangmingyao/yao_dev

Yao dev