Find the direction cosines of the sides of the triangle whose vertices are (3, 5, –4), (–1, 1, 2) and (–5, –5, –2).
Let A(3, 5, – 4), B(–1, 1, 2), C(–5, –5, –2) be the vertices of ΔABC. Direction ratios of AB are – 1 – 3, 1 – 5, 2 + 4 i.e. – 4, – 4, 6 cosines of the line AB as i.e. Direction ratios of BC are – 5 + 1, –5 –1, –2 –2 i.e. – 4, –6, –4. Dividing each by direction ratios of the line BC as Direction ratios of CA are 3+5, 5+5, -4+2 i.e., 8, 10 -2. Dividing each by direction ratios of the line CA as
Last updated at Jan. 25, 2022 by
Question 13 Find the foot of the perpendicular from the point (1, 2, 0) upon the plane x – 3y + 2z = 9. Hence, find the distance of the point (1, 2, 0) from the given plane. Let point P(x1, y1, z1) be foot of perpendicular from point X (1, 2, 0) Since perpendicular to plane is parallel to normal vector Vector (𝑿𝑷) ⃗ is parallel to normal vector 𝒏 ⃗ Given equation of the plane is x − 3y + 2z = 9 So, Normal vector = 𝒏 ⃗ = 𝒊 ̂ − 3𝒋 ̂ + 2𝒌 ̂ Since, (𝑿𝑷) ⃗ and 𝒏 ⃗ are parallel their direction ratios are proportional. Finding direction ratios (𝑿𝑷) ⃗ = (x1 − 1)𝒊 ̂ + (y1 − 2)𝒋 ̂ + (z1 − 0)𝒌 ̂ Direction ratios = x1 − 1, y1 − 2, z1 ∴ a1 = x1 − 1 , b1 = y1 − 2, c1 = z1 𝒏 ⃗ = 1𝒊 ̂ − 3𝒋 ̂ + 2𝒌 ̂ Direction ratios = 1, −3, 2 ∴ a2 = 1 , b2 = −3, c2 = 2 Since, (𝑿𝑷) ⃗ and 𝒏 ⃗ are parallel their direction ratios are proportional. Finding direction ratios Direction ratios are proportional 𝑎_1/𝑎_2 = 𝑏_1/𝑏_2 = 𝑐_1/𝑐_2 = k (𝑥_1 − 1)/1 = (𝑦_1 − 2)/( −3) = 𝑧_1/2 = k Thus, x1 = k + 1, y1 = −3k + 2, z1 = 2k Also, point P(x1, y1, z1) lies in the plane. Putting P (k + 1, −3k + 2, 2k) in equation of plane x − 3y + 2z = 9 (k + 1) − 3(−3k + 2) + 2(2k) = 9 k + 1 + 9k − 6 + 4k = 9 k + 9k + 4k + 1 − 6 = 9 14k − 5 = 9 14k = 9 + 5 14k = 14 ∴ k = 1 Thus, x1 = k + 1 = 1 + 1 = 2 y1 = −3k + 2 = −3(1) + 2 = −1 z1 = 2k = −2(1) = 2 Therefore, coordinate of foot of perpendicular are P (2, −1, 2) Length of perpendicular X (1, 2, 0) and P (2, −1, 2) Let of Perpendicular is length of PX PX = √((2−1)^2+(−1−2)^2+(2−0)^2 ) PX = √(1^2+(−3)^2+2^2 ) PX = √(1+9+4) PX = √𝟏𝟒 units
Last updated at Oct. 27, 2020 by
Question 37 (Choice 2) Find the foot of the perpendicular drawn from the point (−1, 3, −6) to the plane 2𝑥 + 𝑦 − 2𝑧 + 5 = 0. Also find the equation and length of the perpendicular. Let point P(x1, y1, z1) be foot of perpendicular from point X (−1, 3, −6) Since perpendicular to plane is parallel to normal vector Vector (𝑿𝑷) ⃗ is parallel to normal vector 𝒏 ⃗ Given equation of the plane is 2x + y − 2z + 5 = 0 2x + y − 2z = −5 So, Normal vector = 𝒏 ⃗ = 2𝒊 ̂ + 𝒋 ̂ − 2𝒌 ̂ Since, (𝑿𝑷) ⃗ and 𝒏 ⃗ are parallel their direction ratios are proportional. Finding direction ratios (𝑿𝑷) ⃗ = (x1 + 1)𝒊 ̂ + (y1 − 3)𝒋 ̂ + (z1 + 6)𝒌 ̂ Direction ratios = x1 + 1, y1 − 3, z1 + 6 ∴ a1 = x1 + 1 , b1 = y1 − 3, c1 = z1 + 6 𝒏 ⃗ = 2𝒊 ̂ + 𝒋 ̂ − 2𝒌 ̂ Direction ratios = 2, 1, −2 ∴ a2 = 2 , b2 = 1, c2 = −2 Direction ratios are proportional 𝑎_1/𝑎_2 = 𝑏_1/𝑏_2 = 𝑐_1/𝑐_2 = k (𝑥_1 + 1)/2 = (𝑦_1 − 3)/( 1) = (𝑧_1 + 6)/(−2) = k Thus, x1 = 2k − 1, y1 = k + 3, z1 = −2k − 6 Also, point P(x1, y1, z1) lies in the plane. Putting P (2k − 1, k + 3, −2k − 6) in equation of plane 2x + y − 2z = −5 2(2k − 1) + (k + 3) − 2(−2k − 6) = −5 4k − 2 + k + 3 + 4k + 12 = −5 4k + k + 4k − 2 + 3 + 12 = −5 9k + 13 = −5 9k = −5 − 13 9k = −18 ∴ k = −2 Thus, x1 = 2k − 1 = 2(−2) − 1 = −5 y1 = k + 3 = (−2) + 3 = 1 z1 = −2k − 6 = −2(−2) − 6 = −2 Therefore, coordinate of foot of perpendicular are P (−5, 1, −2) Equation of perpendicular Equation of perpendicular would be equation of line joining X (−1, 3, −6) and P (−5, 1, −2) (𝑥 − (−1))/(−5 − (−1))=(𝑦 − 3)/(1 − 3)=(𝑧 − (−6))/(−2 − (−6)) (𝑥 + 1)/(−4)=(𝑦 − 3)/(−2)=(𝑧 + 6)/4 (𝒙 + 𝟏)/(−𝟐)=(𝒚 − 𝟑)/(−𝟏)=(𝒛 + 𝟔)/𝟐 Length of perpendicular X (−1, 3, −6) and P (−5, 1, −2) Let of Perpendicular is length of PX PX = √((−5−(−1))^2+(1−3)^2+(−2−(−6))^2 ) PX = √((−5+1)^2+(−2)^2+(−2+6)^2 ) PX = √((−4)^2+(−2)^2+(4)^2 ) PX = √(16+4+16) PX = √36 PX = 6 units |