Let three vectors overrightarrow a=α i+4 j+2 k, overrightarrow b=5 i+3 j+4 k, overrightarrow =x i+y j+z k form a triangle such that = a- b and the…
JEE Main 2024 — Mathematics Vectors & 3D Geometry
2024mcqeasy
Let three vectors a=αi^+4j^+2k^,b=5i^+3j^+4k^,c=xi^+yj^+zk^ form a triangle such that c=a−b and the area of the triangle is 56. If α is a positive real number, then ∣c∣2 is equal to:
Official previous-year question
Held on 9 Apr 2024 · Verified 6 Jul 2026.
Options
A
16
B
14
C
12
D
10
Did you get this right?
Sign in to track your attempts and accuracy.
Solution
c=a−b⇒(x,y,z)=(α−5,1,−2)⇒x=α−5,y=1,z=−2.....(1)
Area of Δ=56 (given) 21∣a×c∣=56i^αxj^41k^2−2=106⇒∣−10i^−j^(−2α−2x)+k^(α−4x)∣=106⇒(2α+2α−10)2+(α−4α+20)2=500⇒(4α−10)2+(20−3α)2=500⇒25α2−80α−120α=0⇒α(25α−200)=0⇒α=8 (given α is +ve number) ⇒x=α−5=3∣c∣2=x2+y2+z2=9+1+4=14
Your note
Sign in to keep a private note on this question. Nothing you write is ever public.