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Vector Algebra question

2022 · 28 Jun · Shift 1 · Q38
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Vector Algebra question

2022 · 28 Jun · Shift 1 · Q38

JEE MainMathematicsVector AlgebraNumerical+4 / −1
If a→=2i^+j^+3k^\overrightarrow a = 2\widehat i + \widehat j + 3\widehat ka=2i+j​+3k, b→=3i^+3j^+k^\overrightarrow b = 3\widehat i + 3\widehat j + \widehat kb=3i+3j​+k and c→=c1i^+c2j^+c3k^\overrightarrow c = {c_1}\widehat i + {c_2}\widehat j + {c_3}\widehat kc=c1​i+c2​j​+c3​k are coplanar vectors and a→ . c→=5\overrightarrow a \,.\,\overrightarrow c = 5a.c=5, b→⊥c→\overrightarrow b \bot \overrightarrow cb⊥c, then 122(c1+c2+c3)122({c_1} + {c_2} + {c_3})122(c1​+c2​+c3​) is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 150

  1. Let a⃗=(2,1,3),b⃗=(3,3,1),c⃗=(c1,c2,c3).\vec a=(2,1,3),\quad \vec b=(3,3,1),\quad \vec c=(c_1,c_2,c_3).a=(2,1,3),b=(3,3,1),c=(c1​,c2​,c3​).

Given:

  • a⃗,b⃗,c⃗\vec a,\vec b,\vec ca,b,c are coplanar
  • a⃗⋅c⃗=5\vec a\cdot \vec c=5a⋅c=5
  • b⃗⊥c⃗⇒b⃗⋅c⃗=0\vec b\perp \vec c \Rightarrow \vec b\cdot \vec c=0b⊥c⇒b⋅c=0

  1. Since a⃗,b⃗,c⃗\vec a,\vec b,\vec ca,b,c are coplanar and a⃗,b⃗\vec a,\vec ba,b are not parallel, we can write c⃗=xa⃗+yb⃗.\vec c = x\vec a + y\vec b.c=xa+yb.

So, c⃗=x(2,1,3)+y(3,3,1).\vec c = x(2,1,3)+y(3,3,1).c=x(2,1,3)+y(3,3,1).


  1. Use the dot product conditions.

First compute needed dot products: a⃗⋅a⃗=22+12+32=14,\vec a\cdot \vec a = 2^2+1^2+3^2=14,a⋅a=22+12+32=14, a⃗⋅b⃗=2⋅3+1⋅3+3⋅1=12,\vec a\cdot \vec b = 2\cdot 3+1\cdot 3+3\cdot 1=12,a⋅b=2⋅3+1⋅3+3⋅1=12, b⃗⋅b⃗=32+32+12=19.\vec b\cdot \vec b = 3^2+3^2+1^2=19.b⋅b=32+32+12=19.

Now, a⃗⋅c⃗=a⃗⋅(xa⃗+yb⃗)=14x+12y=5...(1)\vec a\cdot \vec c = \vec a\cdot (x\vec a+y\vec b)=14x+12y=5 \quad ...(1)a⋅c=a⋅(xa+yb)=14x+12y=5...(1)

and b⃗⋅c⃗=b⃗⋅(xa⃗+yb⃗)=12x+19y=0...(2)\vec b\cdot \vec c = \vec b\cdot (x\vec a+y\vec b)=12x+19y=0 \quad ...(2)b⋅c=b⋅(xa+yb)=12x+19y=0...(2)


  1. Solve equations (1) and (2):

From (2), 12x=−19y⇒x=−1912y.12x=-19y \Rightarrow x=-\frac{19}{12}y.12x=−19y⇒x=−1219​y.

Substitute into (1): 14(−1912y)+12y=5.14\left(-\frac{19}{12}y\right)+12y=5.14(−1219​y)+12y=5.

−26612y+12y=5-\frac{266}{12}y+12y=5−12266​y+12y=5 −1336y+726y=5-\frac{133}{6}y+\frac{72}{6}y=5−6133​y+672​y=5 −616y=5-\frac{61}{6}y=5−661​y=5 y=−3061.y=-\frac{30}{61}.y=−6130​.

Then x=−1912(−3061)=95122.x=-\frac{19}{12}\left(-\frac{30}{61}\right)=\frac{95}{122}.x=−1219​(−6130​)=12295​.


  1. Now find c⃗\vec cc: c⃗=xa⃗+yb⃗.\vec c=x\vec a+y\vec b.c=xa+yb.

So, c1+c2+c3=x(2+1+3)+y(3+3+1).c_1+c_2+c_3 = x(2+1+3)+y(3+3+1).c1​+c2​+c3​=x(2+1+3)+y(3+3+1).

That is, c1+c2+c3=6x+7y.c_1+c_2+c_3 = 6x+7y.c1​+c2​+c3​=6x+7y.

Substitute x=95122x=\frac{95}{122}x=12295​ and y=−3061=−60122y=-\frac{30}{61}=-\frac{60}{122}y=−6130​=−12260​: c1+c2+c3=6⋅95122+7⋅(−60122).c_1+c_2+c_3 = 6\cdot \frac{95}{122}+7\cdot \left(-\frac{60}{122}\right).c1​+c2​+c3​=6⋅12295​+7⋅(−12260​).

=570122−420122=150122.=\frac{570}{122}-\frac{420}{122}=\frac{150}{122}.=122570​−122420​=122150​.

Therefore, 122(c1+c2+c3)=150.122(c_1+c_2+c_3)=150.122(c1​+c2​+c3​)=150.


  1. Final answer: 150\boxed{150}150​
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