Angle Between Two Planes
3D Geometry
16
JEE Qs
8%
Hard
40
min
Always ensure correct identification of the normal vectors from the plane equations and meticulously apply the dot product formula, remembering to use the absolute value for the acute angle.
๐งฎ Key Formulas
โ Key Points for JEE
- 1The angle between two planes is defined as the acute angle between their normal vectors. Hence, always use the absolute value in the numerator of the cosine formula.
- 2The normal vector to a plane Ax + By + Cz + D = 0 is directly given by n = <A, B, C>. For a plane r . n_vec = d, n_vec is its normal vector.
- 3Understanding the geometric interpretation of the dot product is key, as it directly relates to the angle between the normal vectors.
- 4Special cases like parallel planes (normal vectors are collinear) and perpendicular planes (normal vectors are orthogonal, dot product is zero) are important and simplify calculations.
โ ๏ธ Common Mistakes
- โForgetting to take the absolute value of the dot product in the numerator, which can lead to calculating the obtuse angle instead of the standard acute angle.
- โIncorrectly identifying the normal vectors from the equations of the planes, especially if the plane equations are not in standard form (e.g., if coefficients are misread).
- โMaking computational errors while calculating the dot product or the magnitudes of the normal vectors.
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 Mathematics Ch 11: Three Dimensional Geometry