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+Finished Unit 1, Performing Transformation, in High School Geometry on
+Khan Academy. There are a few kinds of transformations: translation,
+rotate, reflection, and dilation. The first three are rigid
+transformations, but dilation is not. Dilation changes the size of a
+shape by scaling up or down. Rigid transformations only change a shape's
+position while preserving the lengths of all lines and angles.
+
+Translation moves an object up or down, left or right.
+
+Rotation has a center of rotation and an angle that determines how many
+degress the object moves. Positive angles rotate the object
+counterclockwise, while negative angles rotate it clockwise. If the
+origin is the center of rotation and the angle is a multiple of 90°, we
+have some useful patterns:
+
+R(0,0),90°(x,y) = (-y,x)
+R(0,0),180°(x,y) = (-x,-y)
+R(0,0),-90°(x,y) = (y,-x)
+
+Each additional 90° counterclockwise rotation swaps the coordinates and
+negates the new x-coordinate. Clockwise rotation does the same, but
+negates the new y-coordinate instead. In this case, we can determine the
+new position algebraically. Visualization is also useful. We can imagine
+a virtual triangle or rectangle with the point as one of its vertices,
+which helps us understand the rotation more intuitively.
+
+Reflection has a line of reflection that creates a mirror image of the
+original object. Every point or vertex has the same perpendicular
+distance to the reflection line, extending to the opposite side by the
+same distance.
+
+I haven't started Unit 2 yet, but it seems to go deeper into the
+transformations I've learned today, and there is a proof lesson at the
+end of the unit. Let me tackle it tomorrow.
+
+Besides mathematics, I am going to build a Certbot plugin for Alibaba
+Cloud DNS. One of my clients has a project running on Alibaba Cloud, and
+their domain names are also hosted there. Even though there are many
+third-party ACME-compatible clients available that natively support
+Alibaba Cloud, Certbot is still my first choice because of its
+reputation and supply-chain safety. Alibaba Cloud does not have official
+DNS-challenge support for Certbot, so it would be useful to have a
+plugin that allows SSL certificates to be issued automatically.
+
+A year ago I read the complete ACME protocol—RFC 8555—and implemented a
+full SSL certificate issurance and renewal module in PHP, so I should be
+fairly familliar with the workflow. The plugin focuses on manging DNS
+records so the domain names can pass verification by the ACME server.
+
+---
+
+EP-212 文革60周年丨严歌苓、查建英：文革就是中国人的大屠杀
+https://bumingbai.net/2026/05/16/ep-212-wenge01/
+
+I've been listening to the show through Apple Podcasts and didn't even
+know it had a video version. It's a heavy topic for Chinese people, and
+the two guests shared the tragic stories from their youth. It's hard to
+image how people survived those difficult times, when violence and
+irrationality shaped everyone's life.
+
+---
+
+What I've learned about learning a language
+https://petargyurov.com/2022-10-30/learning-spanish
+
+I discovered this blog through Hacker News today, although not because
+of this article specifically. I've also noticed that compared to my
+English reading and writing skills, my speaking and listening are much
+weaker.
+
+Watching YouTube videos and listening to podcasts are my favorite ways
+to improve my listening, but sometimes the episodes are hard to follow,
+and finding material that suits my current level is not easy.
+
+I've tried improving my speaking with ChatGPT serveral times before, and
+I liked the experience. One great advantage is that it never gets
+frustrated and always has the patience to listen to me. All I need is a
+topic to talk about. Maybe the next time I run into a problem in daily
+life, I should talk to it instead of typing on the keyboard.
+
+I also like Duolingo, but the streak system really annoys me. I don't
+need it to feel that I am making progress, and the streaks sometimes put
+pressure on me. I don't think missing a day or two really matters, and
+I wish there were an option to turn it off.