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If A=[a4−5db−65ec]A = \begin{bmatrix} a & 4 & -5 \\ d & b & -6 \\ 5 & e & c \end{bmatrix} is a skew symmetric matrix, then value of a+b+c+d+ea + b + c + d + e is equal to

Solution

✅ Correct Option: 4

A skew-symmetric matrix satisfies AT=−AA^T = -A, which means:

  • All diagonal elements equal 00
  • Element at position (i,j)(i,j) equals negative of element at position (j,i)(j,i)

Given matrix:

A=[a4−5db−65ec]A = \begin{bmatrix} a & 4 & -5 \\ d & b & -6 \\ 5 & e & c \end{bmatrix}


The diagonal elements are aa, bb, and cc.

For a skew-symmetric matrix, all diagonal elements equal 00:

a=0a = 0

b=0b = 0

c=0c = 0


For positions (1,2)(1,2) and (2,1)(2,1):

Element at (1,2)(1,2) is 44

Element at (2,1)(2,1) is dd

Since AT=−AA^T = -A:

d=−4d = -4


For positions (1,3)(1,3) and (3,1)(3,1):

Element at (1,3)(1,3) is −5-5

Element at (3,1)(3,1) is 55

Check: −5=−5-5 = -5 ✓


For positions (2,3)(2,3) and (3,2)(3,2):

Element at (2,3)(2,3) is −6-6

Element at (3,2)(3,2) is ee

Since AT=−AA^T = -A:

e=−(−6)=6e = -(-6) = 6


The sum is:

a+b+c+d+e=0+0+0+(−4)+6=2a + b + c + d + e = 0 + 0 + 0 + (-4) + 6 = 2

Therefore, the value of a+b+c+d+ea + b + c + d + e is equal to 22.

2022: 10 Aug Shift 1

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