I think you are TAIWANteacher ? First time using free MicrosoftWhiteboard video conference teaching online due to covid-19 ! Good starts to introduction to the basic concept of calculus ,invented by Leibnitz/Newton about 350 years ago .Is it too early to exposed calculus to lower secindary school students ? They are still troubled by what is Algebra/symbols ? Transition from Number +-×÷ ,simple algebra/symbol confusing their mind ? 3Blue1Brown made them destroying basic concept of Calculus ! 3b1b good for U-Students already expert in calculus !