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# Online calculation of point oblique line equation

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Generally, in a plane Cartesian coordinate system, if the straight line L passes through points A(X1, Y1) and B(X2, Y2), where x1≠x2, then AB=(x2-x1, y2-y1) is one of L The direction vector, so that the slope of the straight line L is k=(y2-y1)/(x2-x1), and then k=tanα(0≤α<π), the inclination angle α of the straight line L can be obtained. Let tanα=k, The equation y-y0=k(x-x0) is called the point-oblique equation of a straight line, where (x0, y0) is a point on a straight line.

When α is π/2 (90 degrees, the line is perpendicular to the X axis), tanα is meaningless, and there is no point oblique equation.

Point-pointed equations are commonly used in derivatives, using the known tangent line and the derivative of the curve equation (the slope of the tangent to a point on the equation) to find the tangent equation. Applicable to know the coordinates of a point and the slope of the line, and find the title of the line equation.