NCERT Solutions
Class 12 Maths
Three Dimensional Geometry

Ex.Misc.Q.12
Find the coordinates of the point where the line through (3, −4, −5) and (2, −3, 1) crosses the plane 2x + y + z = 7.
It is known that the equation of the line passing through the points, (x1, y1, z1) and (x2,y2, z2) is
Since the line passes through the points, (3, −4, −5) and (2, −3, 1), its equation is given by,
=> (say)
=> x = 3 – k, y = k - 4, z = 6k – 5
Therefore, any point on the line is of the form (3 − k, k − 4, 6k − 5).
This point lies on the plane, 2x + y + z = 7
=> 2(3 − k) + (k − 4) + (6k − 5) = 7
=> 6 – 2k + k – 4 + 6k – 5 = 7
=> 5k - 3 = 7
=> 5k = 10
=> k = 2
Hence, the coordinates of the required point are (3 − 2, 2 − 4, 6 * 2 − 5) i.e., (1, −2, 7).