SECTION 6.4 MATRIX ALGEBRA
THE ALGEBRA OF MATRICES Addition:
PROPERTIES OF MATRICES Commutative: A + B = B + A Associative: A + (B + C) = (A + B) + C
ZERO FOR MATRICES
ADDITIVE INVERSE FOR MATRICES
MORE PROPERTIES OF MATRICES Additive Identity:There is a matrix 0 satisfying 0 + A = A + 0 = A Additive Inverses:For each matrix A, there is a matrix -A satisfying A + (-A) = (-A) + A = 0
MORE PROPERTIES OF MATRICES k(A + B) = kA + kB (k + m)A = kA + mA (km)A = k(mA) = m(kA)
3A = -2B = 3A - 2B =
MULTIPLICATION: aA+bCaB+bD
MULTIPLICATION: aA+bC cA+dC aB+bD cB+dD
Find A B
- 19 Find A B
Find B A
Find B A
MULTIPLICATION: AB = BA = AB BA Multiplication is not commutative.
MORE PROPERTIES OF MATRICES Associative: A (B C) = (A B) C Distributive: A (B + C) = A B + A C (B + C) A = B A + C A
COMPATABILITY OF MATRICES For Addition:Same Size Counterexample:
COMPATABILITY OF MATRICES For Multiplication:
COMPATABILITY OF MATRICES For Multiplication: Two matrices are compatable for multiplication when the dimensions are:n x m and m x p The product matrix will have dimension n x p
USING A CALCULATOR For Multiplication:
INVERSES OF MATRICES With matrices, when AB = I = BA, we say A and B are inverses. Furthermore, when a matrix A has an inverse, we denote it as A -1 Many matrices will fail to have an inverse.
THEOREM: MULTIPLICATIVE INVERSES The matrix has a multiplicative inverse if and only if D = ad - bc is nonzero.
If D 0, then
Does A have an inverse? D = 2(-9) - 5(-5) = = = 7 = 7
Does A have an inverse? D = 3(10) - (-6)(-5) = = = 0 = 0
INVERSES OF MATRICES This method of finding the inverse of a matrix works nicely on 2 x 2 matrices. Finding an inverse of a 3 x 3 matrix is another matter. We’ll use the calculator!
EXAMPLE:
APPLICATIONS TO SYSTEMS OF EQUATIONS 2x + 6y + 6z = 2 2x + 7y + 6z = - 3 2x + 7y + 7z = - 5 =
APPLICATIONS TO SYSTEMS OF EQUATIONS = A - 1 =
SOLVE THE SYSTEM OF EQUATIONS 2x + 6y + 6z = 2 2x + 7y + 6z = - 3 2x + 7y + 7z = - 5 = - 1
SOLVE THE SYSTEM OF EQUATIONS =
CONCLUSION OF SECTION 6.4 CONCLUSION OF SECTION 6.4