In order to calculate the intensity of general-stress in tri-axis test in geotechnical investigation , the author uses cartesian geometry
principle to derivate calculation functions of common tangent points coordinates of every circle by the common tangent line .Especially , the
slope tangent line is calculated directly .With regard to all of tangent points , the linear regression method is used to derive the equation of
the tangent line , and then to calculate the coacervation force Cu and the friction angle φu of the soil.This direct calculating method may
take place of the illustration methods by hands or interactive illustration method on the computer , and is faster and may take more precise .