Join WhatsApp Icon JEE WhatsApp Group
Question 16

Let $$\overrightarrow{a},\overrightarrow{b},\overrightarrow{c}$$ be three vectors such that $$\overrightarrow{a}\times\overrightarrow{b}=2(\overrightarrow{a}\times\overrightarrow{c}).$$ If $$ \mid \overrightarrow{a}\mid, \mid\overrightarrow{b}\mid = 4, \mid \overrightarrow{c}\mid = 2,$$ and the angle between $$\overrightarrow{b}$$ and $$\overrightarrow{c}$$ is $$60^{o}$$, then $$\mid\overrightarrow{a}\cdot\overrightarrow{c}$$ is

Since $$\overrightarrow{a} \times \overrightarrow{b} = 2(\overrightarrow{a} \times \overrightarrow{c})$$, it follows that $$\overrightarrow{a} \times \overrightarrow{b} - 2(\overrightarrow{a} \times \overrightarrow{c}) = \overrightarrow{0}$$ and hence $$\overrightarrow{a} \times (\overrightarrow{b} - 2\overrightarrow{c}) = \overrightarrow{0}$$. This implies that $$\overrightarrow{a}$$ is parallel to $$(\overrightarrow{b} - 2\overrightarrow{c})$$, so one can write $$(\overrightarrow{b} - 2\overrightarrow{c}) = \lambda\,\overrightarrow{a}$$ for some scalar $$\lambda$$.

Taking magnitudes squared of both sides gives $$|\overrightarrow{b} - 2\overrightarrow{c}|^2 = \lambda^2 |\overrightarrow{a}|^2$$, which expands to $$|\overrightarrow{b}|^2 - 4\,\overrightarrow{b}\cdot\overrightarrow{c} + 4|\overrightarrow{c}|^2 = 16\lambda^2$$. Now $$\overrightarrow{b}\cdot\overrightarrow{c} = |\overrightarrow{b}|\,|\overrightarrow{c}|\cos 60° = 4 \times 2 \times \tfrac12 = 4$$, so this becomes $$16 - 16 + 16 = 16\lambda^2 \implies \lambda^2 = 1 \implies \lambda = \pm 1$$.

Next, taking the dot product with $$\overrightarrow{c}$$ in $$(\overrightarrow{b} - 2\overrightarrow{c}) = \lambda\,\overrightarrow{a}$$ yields $$\overrightarrow{a}\cdot\overrightarrow{c} = \frac{(\overrightarrow{b} - 2\overrightarrow{c})\cdot\overrightarrow{c}}{\lambda} = \frac{\overrightarrow{b}\cdot\overrightarrow{c} - 2|\overrightarrow{c}|^2}{\lambda} = \frac{4 - 8}{\lambda} = \frac{-4}{\lambda}$$. When $$\lambda = 1$$, $$\overrightarrow{a}\cdot\overrightarrow{c} = -4$$ and thus $$|\overrightarrow{a}\cdot\overrightarrow{c}| = 4$$; similarly, when $$\lambda = -1$$, $$\overrightarrow{a}\cdot\overrightarrow{c} = 4$$ so again $$|\overrightarrow{a}\cdot\overrightarrow{c}| = 4$$.

The correct answer is Option A: 4.

Get AI Help

Video Solution

video

Create a FREE account and get:

  • Free JEE Mains Previous Papers PDF
  • Take JEE Mains paper tests

JEE Quant Questions | JEE Quantitative Ability

JEE DILR Questions | LRDI Questions For JEE

JEE Verbal Ability Questions | VARC Questions For JEE

Free JEE Topicwise Questions

JEE Rotational MotionJEE Units & MeasurementsJEE Atomic StructureJEE GravitationJEE Periodic Table & PeriodicityJEE StatisticsJEE Inverse Trigonometric FunctionsJEE Magnetism & Magnetic MaterialsJEE Sequences & SeriesJEE MatricesJEE Alternating CurrentsJEE Carboxylic AcidsJEE Permutations & CombinationsJEE Work, Energy & PowerJEE Electromagnetic InductionJEE Electronic DevicesJEE d and f-Block ElementsJEE Chemical KineticsJEE Heat TransferJEE Three Dimensional GeometryJEE Magnetic Effects of CurrentJEE Hydrocarbons - AromaticJEE Electromagnetic WavesJEE Aldehydes & KetonesJEE Hydrocarbons - AlkanesJEE Applications of DerivativesJEE EquilibriumJEE Indefinite IntegrationJEE Chemical ThermodynamicsJEE ElectrochemistryJEE ProbabilityJEE BiomoleculesJEE Continuity & DifferentiabilityJEE Kinetic Theory of GasesJEE Vector AlgebraJEE Hydrocarbons - AlkynesJEE Differential EquationsJEE Current & ResistanceJEE Straight LinesJEE WavesJEE Redox ReactionsJEE Hydrocarbons - AlkenesJEE DeterminantsJEE SolutionsJEE Ray OpticsJEE Dual Nature of Matter & RadiationJEE Chemical Bonding & Molecular StructureJEE Complex NumbersJEE Sets, Relations & FunctionsJEE Electric Charges & FieldsJEE Laws of MotionJEE Fluid MechanicsJEE Basic Concepts in ChemistryJEE Trigonometric FunctionsJEE LimitsJEE Laws of ThermodynamicsJEE Kinematics - 2D MotionJEE p-Block Elements (Groups 13-18)JEE Simple Harmonic MotionJEE Electric Potential & CapacitanceJEE Coordination CompoundsJEE JEE 2D GeometryJEE CirclesJEE Definite IntegrationJEE EMF & Circuit AnalysisJEE Surface TensionJEE Atoms & NucleiJEE Laboratory Experiments - XIJEE Number SystemJEE Basic Principles of Organic ChemistryJEE Wave OpticsJEE Quadratic EquationsJEE Alcohols, Phenols & EthersJEE Organic Compounds with HalogensJEE DifferentiationJEE Conic SectionsJEE Nitrogen-Containing CompoundsJEE ElasticityJEE Practical Organic ChemistryJEE Kinematics - 1D MotionJEE Purification & CharacterisationJEE Binomial Theorem
Ask AI