4.同位角、内错角、同傍内角
https://v.qq.com/txp/iframe/player.html?vid=l0729r24zu1&auto=0
5.平行线
https://v.qq.com/txp/iframe/player.html?vid=u07945duqn7&auto=0
6.平行线的判定
https://v.qq.com/txp/iframe/player.html?vid=b07948abr97&auto=0
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