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Saturday, 31 October

JEE Main 2021 · MathsMultiple choiceSingle correctHardMulti-step

JEE Main 31 August 2021, Shift 2, Maths Q75

Question 75 of 90 in this shift, Maths question 15 of 30, Section A.

Let a⃗,b⃗,c⃗\vec{a}, \vec{b}, \vec{c} be three vectors mutually perpendicular to each other and have same magnitude. If a vector r⃗\vec{r} satisfies a⃗×{(r⃗−b⃗)×a⃗}+b⃗×{(r⃗−c⃗)×b⃗}+c⃗×{(r⃗−a⃗)×c⃗}=0⃗\vec{a}\times\left\{(\vec{r}-\vec{b})\times\vec{a}\right\}+\vec{b}\times\left\{(\vec{r}-\vec{c})\times\vec{b}\right\}+\vec{c}\times\left\{(\vec{r}-\vec{a})\times\vec{c}\right\} = \vec{0}, then r⃗\vec{r} is equal to :
  1. (1)12(a⃗+b⃗+2c⃗)\frac{1}{2}\left(\vec{a}+\vec{b}+2\vec{c}\right)
  2. (2)12(a⃗+b⃗+c⃗)\frac{1}{2}\left(\vec{a}+\vec{b}+\vec{c}\right)Official answer
  3. (3)13(a⃗+b⃗+c⃗)\frac{1}{3}\left(\vec{a}+\vec{b}+\vec{c}\right)
  4. (4)13(2a⃗+b⃗−c⃗)\frac{1}{3}\left(2\vec{a}+\vec{b}-\vec{c}\right)

Official answer

Option 2

NTA final key.