数学必修4第三章 三角恒等变换

https://v.qq.com/txp/iframe/player.html?vid=d01991j25bn&width=500&height=375&auto=0

https://v.qq.com/txp/iframe/player.html?vid=f067399aod5&width=500&height=375&auto=0

https://v.qq.com/txp/iframe/player.html?vid=q050943edy1&width=500&height=375&auto=0