108s1 2007 exam solutions

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MATH 108-07S1 Final Exam - Solutions Q1. [20 marks] (a) [8 marks] The augmented matrix for the system is 1 2 1 1 1 3 2 k 3 6 3 k The row operatio ns R 2 := R 2 R 1 , R 3 := R 3 3R 1 lead to 1 2 1 1 0 1 3 k 1 0 0 0 k 3 So for the system to be consistent, we need k = 3. When k = 3 the matrix is 1 2 1 1 0 1 3 2 0 0 0 0 Solve by back substitution with z = t to obtain y + 3t = 2, i.e. y = 2 3t x + 2(2 3t) t = 1, i.e. x = 3 + 7t i.e. (x,y,z) = (3 + 7t, 2 3t, t). Geometrically, the system describes three planes which intersect in a line. (b) [4 marks] (i) (I A)(I + A 2 ) = I A + A 2 A 2 2 = I A + A 2 + A 2 = I and similarly (I + A 2 )(I A) = I So I A is invertible and (I A) 1 = I + A 2 . (ii) If A is invertible, A 1 (A 2 ) = A 1 A i.e. (A 1 A)A = I I A = I A = I 1

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