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MathsMediumClass 12

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.

๐Ÿงฎ Key Formulas

Let two planes be P1: a1x + b1y + c1z + d1 = 0 and P2: a2x + b2y + c2z + d2 = 0. Their normal vectors are n1 = <a1, b1, c1> and n2 = <a2, b2, c2> respectively.
The angle ฮธ between the two planes is given by cos(ฮธ) = |n1 . n2| / (|n1| |n2|)
In Cartesian form: cos(ฮธ) = |a1a2 + b1b2 + c1c2| / (sqrt(a1^2 + b1^2 + c1^2) * sqrt(a2^2 + b2^2 + c2^2))
If the planes are parallel, then n1 is parallel to n2, i.e., a1/a2 = b1/b2 = c1/c2 (assuming a2, b2, c2 are non-zero).
If the planes are perpendicular, then n1 . n2 = 0, i.e., a1a2 + b1b2 + c1c2 = 0.

โœ… 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.

โš ๏ธ Common Mistakes

  • โœ•Forgetting 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.
  • โœ•Incorrectly 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).
  • โœ•Making computational errors while calculating the dot product or the magnitudes of the normal vectors.

๐Ÿ“ Practice Questions

See all

Q4. Let P be the foot of the perpendicular from the point (1, 2, 2) on the line L : xโˆ’11 = y+1โˆ’1 = zโˆ’22 . Let the line โ†’r = (โˆ’^i + ^j โˆ’2^k) + ฮป(^i โˆ’^j + ^k), ฮป โˆˆR, intersect the line L at Q . Then 2(PQ)2 is equal to : (1) 25 (2) 19 (3) 29 (4) 27

2025ยทMCQMedium

Q16.Let a straight line L pass through the point P(2, โˆ’1, 3) and be perpendicular to the lines xโˆ’12 = y+11 = zโˆ’3โˆ’2 and xโˆ’3 1 = yโˆ’23 = 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) โˆš10 (2) 2โˆš3 (3) 2 (4) 3

2025ยทMCQMedium

Q8. Let L1 : xโˆ’12 = yโˆ’23 = zโˆ’34 and L2 : xโˆ’23 = yโˆ’44 = zโˆ’55 be two lines. Then which of the following points lies on the line of the shortest distance between L1 and L2 ? (1) ( 143 , โˆ’3, 223 ) (2) (โˆ’53 , โˆ’7, 1) (3) (2, 3, 13 ) (4) ( 83 , โˆ’1, 13 )

2025ยทMCQHard

Q25.Let L1 : xโˆ’13 = yโˆ’1โˆ’1 = z+10 and L2 : xโˆ’22 = 0y = z+4ฮฑ , ฮฑ โˆˆR, be two lines, which intersect at the point B. If P is the foot of perpendicular from the point A(1, 1, โˆ’1) on L2 , then the value of 26ฮฑ( PB)2 is _________

2025ยทNumericalMedium

Q14.The perpendicular distance, of the line xโˆ’1 2 = โˆ’1 = z+32 from the point P(2, โˆ’10, 1), is : (1) 6 (2) 5โˆš2 (3) 4โˆš3 (4) 3โˆš5

2025ยทMCQMedium

Q3. Let the position vectors of the vertices A, B and C of a tetrahedron ABCD be ^i + 2^j + ^k,^i + 3^j โˆ’2^k and 2^i + ^j โˆ’^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 โˆš110 and the volume of the 3 tetrahedron is โˆš805 , then the position vector of E is 6โˆš2 (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)

2025ยทMulti conceptHard

NCERT Chapters

  • Class 12 Mathematics Ch 11: Three Dimensional Geometry