That's the heart line. The equation is ρ(θ) = a( 1+cosθ). Christine was a princess in Sweden in the 7th century. She is beautiful, kind and intelligent, and especially likes math. One day, she put on casual clothes and went outside the palace. On the way, she saw many beggars, one of whom was very special. He didn't take the initiative to ask passers-by for charity, but quietly squatted on the ground and concentrated on studying math problems. The man didn't know that the lady standing in front of him was the princess, but he was surprised by the mathematical talent revealed in the lady's words, so he was happy to talk to Christine. Princess Christine realized that he was originally a mathematician, but unfortunately, for some reason, he was frustrated in doing mathematics in France and was down and out, and finally he wandered to Sweden. So Princess Christine invited mathematicians to the palace to be her math teacher, and they discussed math problems together and talked about everything. For a long time, they were immersed in the world of mathematics and love, and they were very happy. But Christine's father knows her daughter's love. The stubborn king doesn't care about mathematics and mathematicians at all. He felt that the French boy was not good enough for his daughter, so he forcibly separated them, drove the mathematician out of the country and never allowed him to set foot in his own country again. He also kept all the letters he wrote to the princess afterwards ... After his lover left, Christine became silent and no longer liked to talk to anyone ... because only that person in the world could communicate with her. When the man returned to France, he was infected with the Black Death and was dying. Before he died, he sent his thirteenth and last letter to his princess, his lover, Christine. This time the king opened the letter, but he couldn't understand what he was writing. Show it to ministers. Ministers can't understand it. I invited many mathematicians to look at it, but I still can't understand it. Finally, the king had no choice but to return the letter to Christine. Christine opened the last letter left by her lover and found that there was only one simple mathematical formula: r = a( 1-sinθ). Yes, others don't understand, but she knows! That's the two-dimensional coordinate they discussed together before. Using algebra to represent the geometric coordinates of a plane, a mathematical problem that no one has studied, is only known by that person and Christine in the world. This is a secret between him and her. So she took out paper and pen and drew a picture according to the mathematical formula ... this is the shape of a heart, which was later called the heart line. He still loves her! He loved her to the death. She knows. She is the only person in the world who knows. For a long time, people thought that Descartes was the one who expressed his feelings with his heart. However, according to textual research, Descartes came to Stockholm in the winter of 1649 at the invitation of Queen Christina (pictured above) to serve as a court philosopher and teach Queen Christina (the queen has ascended the throne, but Descartes has not been expelled). 1650 suffered from pneumonia at the beginning, and died in Sweden in February of the same year (not from the French Black Death). It can be seen that the mathematician in the story is neither Descartes nor others, or the story is just a beautiful lie.