The Story of Mathematics tells the love story of mathematician Descartes. Descartes was born in France on 1596. When the Black Death broke out in continental Europe, he wandered to Sweden, met Princess Christina, a small Swedish principality of 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 fell seriously ill soon after he returned to France. He wrote to the princess every day, but Christina never received a letter from Descartes because she was intercepted by the king. Descartes died after sending Christina the thirteenth letter, which contained only a short formula: the formula of heart line. The king couldn't understand that they didn't always talk about love, so he gave this letter to Christina, who had been unhappy. 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, and the figure of the equation is a heart. This is also the famous heart line.