THE UNIVERSITY OF SYDNEY Math1901 Differential Calculus (Advanced) Semester 1 Tutorial Week 3 2015 1. (This question is a preparatory question and should be attempted before the tutorial. Answers are provided at the end of the sheet – please check your work.) Express the following complex numbers in Cartesian form: (a) 2cis π4 (b) −4cis π3 (c) cis π2 cis π3 cis π6 (d) e−iπ (e) eln 2+iπ (f) e1+i e1−i e−2−iπ Questions for the tutorial 2. Solve the following equations (leaving your answers in polar form) and plot the solutions in the complex plane: √ √ (a) z 5 = 1 (b) z 6 = −1 (c) z 3 + i = 0 (d) z 4 = 8 2 + 8 2 i 3. The complex sine and cosine functions are defined by the formulas sin z = eiz − e−iz , 2i cos z = eiz + e−iz , 2 z ∈ C. (a) Show that when z is real, sin z and cos z reduce to the familiar real sine and cosine functions. (b) Show that cos2 z + sin2 z = 1 for all z ∈ C. (c) Show that cos(z + w) = cos z cos w − sin z sin w for all z, w ∈ C. (d) Is it true that | sin z| ≤ 1 and | cos z| ≤ 1, for all z ∈ C? 4. Find all solutions of the following equations: (a) ez = i (b) ez = −10 √ (c) ez = −1 − i 3 (d) e2z = −i 5. Let f : C → C be the function z 7→ z 2 . Sketch the following sets, and then sketch their images under the function f . (a) A = {z ∈ C | Im(z) = 2} (b) B = {z ∈ C | Im(z) = 2Re(z)} (c) C = {z ∈ C | |z| = 1 and Im(z) ≥ 0} (d) D = {z ∈ C | |z| = 1} 6. Sketch the following sets and their images under the function z 7→ ez : (a) A = {z ∈ C | 0 < Re(z) < 2, Im(z) = π/2} (b) B = {z ∈ C | Re(z) = 1, |Im(z)| < π/2} (c) C = {z ∈ C | Re(z) < 0, π/3 < Im(z) < π} (d) D = {z = (1 + i)t | t ∈ R} 7. (a) Sketch the set A = {z ∈ C | 1/2 < |z| < 4, 0 ≤ Arg(z) ≤ π/4}. (b) Sketch the image of A in the w-plane under the function z 7→ 1/z. (c) An insect is crawling clockwise around the boundary of A in the z-plane. Is its image (in the w-plane) crawling clockwise, or anticlockwise? (If clockwise, we say the function is orientation-preserving; if anticlockwise, we say it is orientation-reversing.) (d) Now consider the function z 7→ z¯. Is this orientation-preserving or orientation-reversing? 8. Find all solutions of the equation e2z − (1 + 3i)ez + i − 2 = 0. Extra Questions 9. Solve the following equations for z. (a) z 5 + z 3 − z 2 − 1 = 0, given that z = i is a solution. (b) z 6 − 4z 5 + 8z 4 − 12z 3 + 5z 2 + 40z − 50 = 0, given that z = 2 + i is a solution. 10. There is a ‘cubic formula’ analogous to the much loved quadratic formula, although it is a lot more complicated. In this question we start with a warm-up example, before solving the general cubic equation. (a) Show that for any complex number z, there exists a nonzero complex number w such that w + 1/w = z. (b) Use this substitution to solve the equation z 3 − 3z − 1 = 0. (c) Generalise the above technique to solve the general cubic az 3 + bz 2 + cz + d = 0. Warning: This is a bit labour intensive! It is best to try it outside of tutorial time. Hint: Try a substitution of the form z = u + α, with α to be determined, to reduce the equation to the form u3 + pu − q = 0. Now attempt a substitution of the form u = w + β/w with a cleverly chosen β to reduce the equation to a quadratic in w3 . You can now solve this equation, hence back-track to find z. 11. Let n be a given positive integer. By a primitive nth root of unity we mean a solution η of z n = 1 which has the property that its powers η, · · · , η n−1 , η n (= 1) are exactly the 2π solutions of this equation in C. For example, ei n is a primitive nth root of unity. (a) Find all primitive 6th roots of unity. (b) Find all primitive 5th roots of unity. (c) For which values of k, 0 ≤ k ≤ n − 1, is ei 2πk n a primitive nth root of unity? Solution to Question 1 1. (a) 2cis π/4 = √ 2+ √ 2i √ (b) −4cis π/3 = −2 − 2 3 i (c) cis (π/2) cis (π/3) cis (π/6) = cis π = −1 (e) eln 2+iπ = eln 2 cis π = −2 (d) e−iπ = cis (−π) = −1 (f) e1+i e1−i e−2−iπ = e1+i+1−i−2−iπ = e−iπ = −1

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