Descartes was born in France on 1596. When the Black Death broke out in continental Europe, he wandered to Sweden, where he met the little princess Christina, who was 18 years old, and later became her math teacher. Getting along with each other every day makes them fall in love. The princess's father, the king, flew into a rage and ordered Descartes to be executed. Later, she was exiled to France because of her daughter's intercession, and Princess Christine was also under house arrest by her father. Descartes contracted the Black Death shortly after he returned to France. He writes to the princess every day. Because she was intercepted by the king, Christine never heard from Descartes again. Descartes died after sending the 13 letter to Christine. The letter 13 only contains a short formula: r=a( 1-sinθ). The king couldn't understand that they didn't always talk about love, so he gave this letter to Christine, who had been depressed. As soon as the princess saw it, she immediately understood the intention of her lover. She immediately began to draw a diagram of the equation. She was very happy when she saw the chart. She knows that her lover still loves her. The graph of the equation is a heart. The princess established a polar coordinate system on paper, drew the points of the equation on it with a pen, saw the heart line represented by the equation, and understood Descartes' deep love for herself. This is also the famous "heart line".
(The advertisement of Baisui Mountain mineral water used this idea. )
It's not easy to learn math well, and it's a good way to copy math newspapers by hand. The following are the pictures and materials I b