We will discuss here about the meaning of equation of a straight line.
Let the straight line be PQ which passes through the origin (0, 0) and inclined at 45° with the positive direction of the xaxis. Let the points on the line PQ are (x\(_{1}\), y\(_{1}\)), (x\(_{2}\), y\(_{2}\)), (x\(_{3}\), y\(_{3}\)), etc.,
According to the definition of coordinates \(\frac{y_{1}}{x_{1}}\) = tan 45° = \(\frac{y_{2}}{x_{2}}\) = \(\frac{y_{3}}{x_{3}}\) = etc.,
Therefore, y\(_{1}\) = x\(_{1}\), y\(_{2}\) = x\(_{2}\), y\(_{3}\) = x\(_{3}\), etc.,
Thus, from the above explanation we conclude that for any point (x, y) on the line,
ycoordinate = xcoordinate
i.e., x = y, where (x, y) is any point on the line.
y = x is the equation of the straight line PQ.
Definition of the equation of a straight line:
The equation of a straight line is the common relation between the xcoordinate and ycoordinate of any point on the line.
Note: The coordinates of any point on the straight line satisfy the equation of the line.
Let the equation of a straight line y = 5x  2. The point (1, 3) lies on the line y = 5x 2 because (1, 3) satisfy the equation y = 5x – 2. Since by plugging 1 for x and 3 for y in the equation, we get 3 = 5(1) – 2 i.e., ⟹ 3 = 5 – 2 ⟹ 3 = 3, which is true. But the point (2, 4) does not a lies on the line y = 5x 2 because (2, 4) does not satisfy the equation y = 5x – 2. Since by plugging 2 for x and 4 for y in the equation, we get 4 = 5(2) – 2 i.e., ⟹ 4 = 10 – 2 ⟹ 4 = 8, which is not true.
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