Source: https://www.researchgate.net/profile/Felix-Martinez-De-La-Rosa/publication/310774663_La_recta_tangente_notas_historicas_y_actividades_para_el_aula/links/5836d19808aed45931c805a1/La-recta-tangente-notas-historicas-y-actividades-para-el-aula.pdf