You can find the quadratic equation in the background of the Mona Lisa

Leonardo da Vinci not only had talents in art but the maths and sciences as well, and often found himself with a lack of parchment for all of his studies. Being the resourceful person he is, he often reused many of his unneeded scientific parchments for art canvases. While he was for the most part thorough with completely covering the old notations, due to the paint eroding over time, it has recently been discovered that Da Vinci's studies of the quadratic equation have become slightly visible in the background of his famous Mona Lisa.