Consider a Δ ABC where A(1,3,2),B(-2,8,0) and C(3,6,7). If the angle bisector of angle BAC meets the line BC at D, then the length of the projection…
JEE Main 2024 — Mathematics Vectors & 3D Geometry
2024mcqeasy
Consider a ΔABC where A(1,3,2),B(−2,8,0) and C(3,6,7). If the angle bisector of ∠BAC meets the line BC at D, then the length of the projection of the vector AD on the vector AC is:
Official previous-year question
Held on 1 Feb 2024 · Verified 6 Jul 2026.
Options
A
23837
B
238
C
23839
D
19
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Solution
Given,
A(1,3,2),B(-2,8,0)&C(3,6,7)
And angle bisector of ∠BAC meets the line BC at D,
Now, plotting the diagram we get,
Now, finding
AC=2i^+3j^+5k^
⇒∣AC∣=4+9+25=38
And ∣AB∣=9+25+4=38
Using angle bisector theorem, we get
CDBD=ACAB
⇒BD:CD=1:1
Using section formula to find coordinates of D,
⇒D≡(1+21×3+1(−2),1+21×6+1×8,1+21×7+1×0)
⇒D≡(21,7,27)
⇒AD=−21i^+4j^+23k^
⇒AD=21(−i^+8j^+3k^)
Length of projection of AD on AC is given by,
⇒d=∣ACAD⋅AC∣
⇒d=23837
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