R = A (1-sinθ) This function has two variables, which can be assigned to A and then solved. The function image is a heart-shaped line. This equation is also called "Cartesian love coordinate formula".

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Descartes became the princess's math teacher. Under the careful guidance of Descartes, the princess's mathematics advanced by leaps and bounds, and they began to become close. They are inseparable every day, which makes them fall in love, and a pure and beautiful love quietly sprouts. However, it didn't take long for their love affair to reach the king's ears. In the past, they were furious and ordered Descartes to be executed immediately. At Christine's pleading, the king exiled him to China, and the princess was placed under house arrest.

Descartes, who was weak, fell ill soon after he returned to France. In the countdown to life, he misses the princess day and night, and keeps writing to her every day, expecting her reply. After Descartes sent Christine the thirteenth letter, he left this world forever. There is not a word written in this last letter, but only one equation: r=a( 1-sinθ). This curve is the famous "heart shape".