Let a⃗ = −î + 2ĵ + 2k̂, b⃗ = 8î + 7ĵ − 3k̂ and c⃗ be a vector such that a⃗ × c⃗ = b⃗. If c⃗ · (î + ĵ + k̂) = 4, then |a⃗ + c⃗|² is equal to
🚀 Stop paying for expensive JEE/NEET Coaching! Get 100,000+ Premium Practice Questions for Free. No Subscriptions, Only Quality Learning.
Q. Let \(\vec{a} = -\hat{i} + 2\hat{j} + 2\hat{k}\), \(\vec{b} = 8\hat{i} + 7\hat{j} - 3\hat{k}\) and \(\vec{c}\) be a vector such that \(\vec{a} \times \vec{c} = \vec{b}\). If \(\vec{c} \cdot (\hat{i} + \hat{j} + \hat{k}) = 4\), then \(|\vec{a} + \vec{c}|^2\) is equal to :

(A) 30

(B) 33

(C) 27

(D) 35

Correct Answer: (C) 27

Step-by-Step Detailed Solution

1. Analyze the given vectors:
\(\vec{a} = -\hat{i} + 2\hat{j} + 2\hat{k} \implies |\vec{a}|^2 = 1 + 4 + 4 = 9\)
\(\vec{b} = 8\hat{i} + 7\hat{j} - 3\hat{k} \implies |\vec{b}|^2 = 64 + 49 + 9 = 122\)

2. Use the Vector Property:
Given \(\vec{a} \times \vec{c} = \vec{b}\). We know \(\vec{b} \perp \vec{a}\).
Check: \(\vec{a} \cdot \vec{b} = (-1)(8) + (2)(7) + (2)(-3) = -8 + 14 - 6 = 0\).

3. Find vector \(\vec{c}\) components:
Let \(\vec{c} = x\hat{i} + y\hat{j} + z\hat{k}\).
Determinant form of \(\vec{a} \times \vec{c} = \vec{b}\):
\((2z-2y)\hat{i} + (z+2x)\hat{j} + (-y-2x)\hat{k} = 8\hat{i} + 7\hat{j} - 3\hat{k}\).
Equating parts: \(z-y=4\), \(z+2x=7\), \(y+2x=3\).

4. Solve using dot product constraint:
Given \(\vec{c} \cdot (\hat{i} + \hat{j} + \hat{k}) = 4 \implies x + y + z = 4\).
Substituting relations: \(x = 2, y = -1, z = 3\).
\(\vec{c} = 2\hat{i} - \hat{j} + 3\hat{k} \implies |\vec{c}|^2 = 4 + 1 + 9 = 14\).

5. Final Calculation:
\(\vec{a} \cdot \vec{c} = (-1)(2) + (2)(-1) + (2)(3) = 2\).
\(|\vec{a} + \vec{c}|^2 = |\vec{a}|^2 + |\vec{c}|^2 + 2(\vec{a} \cdot \vec{c})\)
\(|\vec{a} + \vec{c}|^2 = 9 + 14 + 2(2) = 27\).

Related Theory: Advanced Vector Mechanics

Vector Algebra is not just about formulas; it is about visualizing directions in 3D space. While many students opt for expensive JEE Main coaching, these concepts can be mastered through consistent self-study using quality resources.

1. The Geometry of the Cross Product

The cross product \(\vec{a} \times \vec{c}\) generates a third vector perpendicular to both. This is fundamental in Physics for calculating Torque or Magnetic Force. If you are aiming for B.Tech Admissions in top IITs, speed in solving these determinants is essential.

2. Scalar Triple Product & Its Applications

The volume of a parallelepiped formed by vectors is represented by \([\vec{a} \vec{b} \vec{c}]\). This is a frequent topic in JEE Advanced Mathematics. Understanding the coplanarity condition (Volume = 0) is a major rank-booster.

3. Why Self-Study Beats Traditional Coaching

Modern exams like JEE 2026 require analytical thinking rather than rote learning. By using free online study materials and digital mock tests, students can save lakhs of rupees on coaching fees while getting access to superior explanations and global practice standards. Scholarships for Engineering students often favor those who have a deep conceptual grasp rather than just formula memorization.

4. Component Method vs. Identity Method

In this problem, we used the component method. However, Lagrange's Identity: \(|\vec{a} \times \vec{c}|^2 + (\vec{a} \cdot \vec{c})^2 = |\vec{a}|^2 |\vec{c}|^2\) is a much faster route when individual components aren't needed. Online JEE coaching often overlooks these time-saving hacks.

5. Vector Projections and Unit Vectors

A unit vector along any direction is \(\hat{u} = \frac{\vec{v}}{|\vec{v}|}\). In engineering applications like Robotics or Structural Engineering, unit vectors are used to resolve forces in multiple directions. High-percentile students prioritize these fundamental applications.

6. Common Mistakes in Vector Algebra

  • Sign Errors: Forgetting that \(\vec{a} \times \vec{b} \neq \vec{b} \times \vec{a}\).
  • Dot Product Confusion: Mistaking \(\vec{i} \cdot \vec{i} = 1\) with \(\vec{i} \times \vec{i} = 0\).
  • Calculation Pace: Spending too much time on simple determinants. Free test series can help you improve this.

7. Strategic Preparation for JEE 2026

Focus on high-weightage topics: Vectors, 3D Geometry, and Calculus. Using Premium Study abroad scholarships as a goal can motivate students to score in the 99.9 percentile range. Remember, the best B.Tech programs require not just hard work, but smart work.

Related JEE Main Questions

Frequently Asked Questions (FAQs)

1. Why choose self-study over JEE coaching? Self-study allows you to focus on your weak areas and use free, high-quality resources without the high cost of traditional institutes.
2. Is Vector Algebra hard for JEE Main? No, it is one of the most scoring chapters if you understand the geometric meaning of dot and cross products.
3. How to calculate vector magnitude quickly? Square the components, add them, and take the square root. Practice daily to do this mentally.
4. What is the best free resource for JEE 2026? Websites like neetjeerankers.com offer premium-level solutions and practice papers completely for free.
5. Can I get a full B.Tech scholarship? Yes, many top universities provide 100% scholarships to students with high JEE Main rankings.
6. What is the angle between perpendicular vectors? The angle is exactly 90 degrees, and their dot product is always zero.
7. How to manage time in JEE Mathematics? Practice shortcuts like Lagrange's Identity to solve vector problems in under 60 seconds.
8. Are vectors important for NEET? Yes, vectors are crucial for understanding Physics topics like Kinematics and Electromagnetism in the NEET syllabus.
9. What is the unit vector of a? It is the vector divided by its magnitude: \(\vec{a}/|\vec{a}|\).
10. Where can I find more JEE Main PYQs? You can access a massive database of previous year questions on our portal for free.
J

Expert Contribution by: JEE NEET Experts

Specialists in Mathematics & Engineering Entrance Mentorship. 10+ years experience.

Related Covered Topics

JEE Main 2026Free JEE Study MaterialVector Algebra FormulasB.Tech AdmissionsIIT JEE ScholarshipsOnline Coaching AlternativesMathematics ShortcutsEngineering Entrance PrepVector Cross ProductDot Product RulesJEE Main PYQsCompetitive Exam HacksIIT JEE MathematicsNEET Physics VectorsSelf Study GuidesEngineering Entrance 2026
Scroll to Top