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Let the three sides of a triangle ABC be given by the vectors 2hatmathbfi - hatmathbfj + hatmathbfk , hatmathbfi - 3hatmathbfj - 5hatmathbfk and 3hatmathbfi - mathbf4hatmathbfj - mathbf4hatmathbfk . Let G be the centroid of the triangle ABC. Then 6leftleft|overlineAGright|^2 + left|overlineBGright|^2 + left|overlineCGright|^2right) is equal to

Numerical Answer Type:
Enter a numerical value Answer: 164 to 164 +4 marks

Solution & Explanation

### Core Logic Let the vertices of the triangle be A, B, and C. The vectors representing the side paths are: overlineAB = 2hati - hatj + hatk overlineCA = hati - 3hatj - 5hatk overlineCB = 3hati - 4hatj - 4hatk Notice that overlineAB + overlineCA = (2+1)hati + (-1-3)hatj + (1-5)hatk = 3hati - 4hatj - 4hatk = overlineCB. This structurally validates vector addition rules.
Vector algebra diagram for Q75 - JEE Main 2025 Evening
Vector algebra diagram for Q75 - JEE Main 2025 Evening
### Step 1: Finding Position Vectors relative to A Let's set vertex A as the origin origin point (Position vector vecA = vec0): - Position vector of B: vecB = 2hati - hatj + hatk - Position vector of C: Since overlineCA = vecA - vecC = -vecC implies vecC = -hati + 3hatj + 5hatk Now, calculate the position vector of the centroid G: vecG = fracvecA + vecB + vecC3 = fracvec0 + (2hati - hatj + hatk) + (-hati + 3hatj + 5hatk)3 = frac13left(hati + 2hatj + 6hatkright) ### Step 2: Calculating Squared Lengths to the Centroid Let's find each individual vector distance block: - overlineAG = vecG - vecA = frac13(hati + 2hatj + 6hatk) implies |overlineAG|^2 = frac19(1^2 + 2^2 + 6^2) = frac419 - overlineBG = vecG - vecB = left(frac13-2right)hati + left(frac23+1right)hatj + left(2-1right)hatk = -frac53hati + frac53hatj + 1hatk |overlineBG|^2 = left(-frac53right)^2 + left(frac53right)^2 + 1^2 = frac259 + frac259 + 1 = frac599 - overlineCG = vecG - vecC = left(frac13+1right)hati + left(frac23-3right)hatj + left(2-5right)hatk = frac43hati - frac73hatj - 3hatk |overlineCG|^2 = left(frac43right)^2 + left(-frac73right)^2 + (-3)^2 = frac169 + frac499 + 9 = frac1469 ### Step 3: Final Targeted Evaluation Summing the squared values and multiplying by 6: textValue = 6 left[ |overlineAG|^2 + |overlineBG|^2 + |overlineCG|^2 right] = 6 left[ frac419 + frac599 + frac1469 right] textValue = 6 times frac2469 = 2 times frac2463 = 2 times 82 = 164 ### Pattern Recognition Setting one vector node as the origin point (vecA = vec0) heavily dampens intermediate coordinate math steps, avoiding dealing with an absolute baseline origin orientation. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 12 Mathematics: Vector Algebra

Reference Study Guides

More Vector Algebra Previous-Year Questions — Page 4

Q55 2025 Vector Cross Product and Dot Product
Let veca = 2hati - hatj + 3hatk , vecb = 3hati - 5hatj + hatk and vecc be a vector such that veca times vecc = vecc times vecb and left(vecmathbfa + vecmathbfcright) . left(vecmathbfb + vecmathbfcright) = 168. Then the maximum value of |vecmathbfc|^2 is:
  • A. 77
  • B. 462
  • C. 308
  • D. 154

Solution

### Related Formula vecu times vecv = -vecv times vecu textIf vecu times vecv = 0 implies vecu parallel vecv implies vecu = lambda vecv ### Core Logic Given veca times vecc = vecc times vecb implies veca times vecc + vecb times vecc = 0 (veca + vecb) times vecc = 0 implies vecc = lambda(veca + vecb) ### Step 1: Compute a + b veca + vecb = (2+3)hati + (-1-5)hatj + (3+1)hatk = 5hati - 6hatj + 4hatk vecc = lambda(5hati - 6hatj + 4hatk) |vecc|^2 = lambda^2(25 + 36 + 16) = 77lambda^2 ### Step 2: Expand the Dot Product Condition (veca + vecc) cdot (vecb + vecc) = 168 veca cdot vecb + vecc cdot (veca + vecb) + |vecc|^2 = 168 Evaluate veca cdot vecb = (2)(3) + (-1)(-5) + (3)(1) = 6 + 5 + 3 = 14. Substitute vecc cdot (veca + vecb) = lambda |veca + vecb|^2 = 77lambda: 14 + 77lambda + 77lambda^2 = 168 implies 77lambda^2 + 77lambda - 154 = 0 lambda^2 + lambda - 2 = 0 implies lambda = 1 text or lambda = -2 ### Step 3: Maximize |vecc|^2 Maximum value occurs when lambda = -2: |vecc|^2 = 77(-2)^2 = 77 times 4 = 308 ### Pattern Recognition Recognize the cross-product rule inversion immediately: vecx times vecy = vecy times vecz implies (vecx+vecz) parallel vecy. This linear reduction circumvents solving complex linear systems. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 12 Mathematics: Vector Algebra

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