Properties of Adjoint, Inverse & Determinants
Duration: 8 min
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This video lecture focuses on the properties of adjoint and inverse matrices within the context of linear algebra. The instructor begins by listing fundamental properties on a whiteboard, including the relationship between the determinant of a matrix and its inverse. He then provides a step-by-step derivation for the determinant of the adjoint matrix, utilizing scalar multiplication rules. The lesson concludes by cataloging additional properties involving matrix products, powers, scalar multiplication, and transposes, supported by a 2x2 matrix example.
Chapters
0:00 – 2:00 00:00-02:00
The video opens with a title card 'LINEAR ALGEBRA' and a 2x2 determinant formula. The scene shifts to a whiteboard titled 'PROPERTIES OF ADJOINT & INVERSE'. The instructor introduces the first property: $|A^{-1}| = rac{1}{|A|} = |A|^{-1}$, illustrating it with the example $1/2 = 2^{-1}$. He then states that for a singular matrix where $|A|=0$, the inverse $A^{-1}$ does not exist, marking this with a check. He introduces the third property: $|adj(A)| = |A|^{n-1}$ for an $n imes n$ matrix. He extends this to iterated adjoints, writing $|adj(adj(A))| = |A|^{(n-1)^2}$ and $|adj(adj(adj(A)))| = |A|^{(n-1)^3}$. He points to the first property formula while explaining. He also circles the term $|A|$ in the denominator.
2:00 – 5:00 02:00-05:00
The instructor moves to a derivation on the left side of the board to prove $|adj(A)| = |A|^{n-1}$. He starts with the definition $A^{-1} = rac{adj(A)}{|A|}$ and rearranges it to $|A|A^{-1} = adj(A)$. Taking the determinant of both sides gives $||A|A^{-1}| = |adj(A)|$. He applies the scalar multiplication property $|kA| = k^n|A|$, where $k=|A|$, resulting in $|A|^n |A^{-1}| = |adj(A)|$. Substituting $|A^{-1}| = 1/|A|$, he simplifies the left side to $|A|^n \cdot rac{1}{|A|} = |A|^{n-1}$, confirming the property. On the right side, he derives the formula for the determinant of the adjoint of the adjoint. Letting $adj(A) = B$, he uses the property $|adj(B)| = |B|^{n-1}$. Substituting back, he gets $|adj(adj(A))| = |adj(A)|^{n-1}$. Finally, substituting the previous result, he arrives at $|adj(adj(A))| = (|A|^{n-1})^{n-1} = |A|^{(n-1)^2}$. He notes that similarly, the triple adjoint yields $|A|^{(n-1)^3}$. He writes 'Similarly' to indicate the pattern continues.
5:00 – 8:15 05:00-08:15
The final section lists additional properties of determinants and adjoints. Property 4 states $|AB| = |A||B|$, and Property 5 states $|A^k| = |A|^k$. He lists Property 6 as $adj(0) = 0$ (zero matrix) and Property 7 as $adj(I) = I$. Property 8 is $adj(kA) = k^{n-1} adj(A)$, and Property 9 is $adj(A^T) = (adj(A))^T$. On the right side, he displays a 2x2 matrix example $A = egin{bmatrix} a & b \ c & d \end{bmatrix}$, showing its adjoint $adj(A) = egin{bmatrix} d & -b \ -c & a \end{bmatrix}$ and inverse $A^{-1} = rac{1}{ad-bc} egin{bmatrix} d & -b \ -c & a \end{bmatrix}$. He points to the formula $|AB| = |A||B|$ and the scalar multiplication property $adj(kA) = k^{n-1} adj(A)$ while explaining. The video concludes with these fundamental properties written on the board. He also circles 'mxn matrix' in the previous section.
The lecture systematically builds understanding of matrix properties, starting with the relationship between a matrix and its inverse determinant. It then rigorously derives the determinant of the adjoint matrix using scalar multiplication rules. The lesson concludes by cataloging essential properties for products, powers, and transposes, providing a comprehensive reference for solving linear algebra problems involving determinants and adjoints.