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Inverse of a 2×2

EasyLinear Algebra~3m

The inverse of (3124)\begin{pmatrix} 3 & 1 \\ 2 & 4 \end{pmatrix} is 1ad−bc(d−b−ca)\tfrac{1}{ad-bc}\begin{pmatrix} d & -b \\ -c & a \end{pmatrix}. What is its top-left entry?

Show hints (2)+
  1. Swap the diagonal, negate the off-diagonal, divide by $\det$.
  2. $\det = 10$ here.

Answer

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0.4 (± 0.01)

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Asked at: Citadel, Two Sigma

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