>>> coeffs_secantes(f,-5,5,25)
[-11.6, -10.8, -10.0, -9.2, -8.4, -7.6, -6.8, -6.0, -5.2, -4.4, -3.6, 
-2.8, -2.0, -1.2, -0.4, 0.4, 1.2, 2.0, 2.8, 3.6, 4.4, 5.2, 6.0, 6.8, 7.6]