Image of Point in Plane
3D Geometry
16
JEE Qs
8%
Hard
60
min
Master the step-by-step derivation using line and plane equations; this approach is more robust for variations than just memorizing the direct formula.
๐งฎ Key Formulas
โ Key Points for JEE
- 1The image of a point P in a plane is a point P' such that the plane is the perpendicular bisector of the line segment PP'.
- 2The foot of the perpendicular (M) from P to the plane is the midpoint of the line segment PP'.
- 3The line connecting the point P and its image P' (i.e., PP') is always normal (perpendicular) to the given plane. Therefore, its direction ratios are proportional to the coefficients of x, y, z in the plane's equation.
- 4The process to find the image involves: 1) Writing the equation of the line normal to the plane and passing through the given point. 2) Finding the intersection point (foot of perpendicular) of this line and the plane. 3) Using the midpoint formula (foot is midpoint of P and P') to find the image P'.
โ ๏ธ Common Mistakes
- โConfusing the formula for the image of a point with the formula for the foot of the perpendicular (the image formula has -2, foot of perpendicular has -1).
- โSign errors when substituting coordinates or the constant 'd' into the formula for image or foot of perpendicular.
- โIncorrectly identifying the direction ratios of the normal vector to the plane, especially if the plane equation is not in standard form (e.g., if coefficients are zero).
๐ Practice Questions
See allQ4. Let P be the foot of the perpendicular from the point (1, 2, 2) on the line L : xโ11 = y+1โ1 = zโ22 . Let the line โr = (โ^i + ^j โ2^k) + ฮป(^i โ^j + ^k), ฮป โR, intersect the line L at Q . Then 2(PQ)2 is equal to : (1) 25 (2) 19 (3) 29 (4) 27
Q16.Let a straight line L pass through the point P(2, โ1, 3) and be perpendicular to the lines xโ12 = y+11 = zโ3โ2 and xโ3 1 = yโ23 = z+24 . If the line L intersects the yz -plane at the point Q , then the distance between the points P and Q is : (1) โ10 (2) 2โ3 (3) 2 (4) 3
Q8. Let L1 : xโ12 = yโ23 = zโ34 and L2 : xโ23 = yโ44 = zโ55 be two lines. Then which of the following points lies on the line of the shortest distance between L1 and L2 ? (1) ( 143 , โ3, 223 ) (2) (โ53 , โ7, 1) (3) (2, 3, 13 ) (4) ( 83 , โ1, 13 )
Q25.Let L1 : xโ13 = yโ1โ1 = z+10 and L2 : xโ22 = 0y = z+4ฮฑ , ฮฑ โR, be two lines, which intersect at the point B. If P is the foot of perpendicular from the point A(1, 1, โ1) on L2 , then the value of 26ฮฑ( PB)2 is _________
Q14.The perpendicular distance, of the line xโ1 2 = โ1 = z+32 from the point P(2, โ10, 1), is : (1) 6 (2) 5โ2 (3) 4โ3 (4) 3โ5
Q3. Let the position vectors of the vertices A, B and C of a tetrahedron ABCD be ^i + 2^j + ^k,^i + 3^j โ2^k and 2^i + ^j โ^k respectively. The altitude from the vertex D to the opposite face ABC meets the median line segment through A of the triangle ABC at the point E . If the length of AD is โ110 and the volume of the 3 tetrahedron is โ805 , then the position vector of E is 6โ2 (1) 12 1 (7^i + 4^j + 3^k) (2) 12 (^i + 4^j + 7^k) (3) 1 6 (12^i + 12^j + ^k) (4) 16 (7^i + 12^j + ^k)
NCERT Chapters
- Class 12 Maths Ch 11: Three Dimensional Geometry