1. Use the complex exponential to evaluate d31 dr1 (cosh(x) sin(x)). dx31 2. (a) Sketch the graph of y = a sech (b(x c)) + d, where a, b, c, d are positive constants. Remember to label all key features with their values, including the asymptotes and local maximu. (b) During a flood, river levels often approximately follow a bell curve, which can be modelled using sech. The Murray River is one of Australia's largest rivers. It forms part of the border between Victoria and NSW. Between September 2022 and February 2023, the Murray River was in flood. Many towns were affected, including Echuca, about 300km north of Melbourne. Normally, the Murray River water level at Echuca is 86.9 metres above sea level. At the flood's peak, the water level was 95.0 metres above sea level, and this occurred on 27 October 2022. 58 days later (24 December), the water level was 90.3 metres above sea level. Lett be the time in days since 31 December 2021 (so t = 1 represents 1 January 2022) and let y(t) be the Murray River water level at Echuca in metres above sea level at time t. Assume that y(t) = a sech (b(t c)) + d - where a, b, c, d are positive constants. Use the information above to find the values of a, b, c and d. You can give your answers as decimals to at least 4 decimal places. You can use a calculator or software like Wolfram Alpha wolframalpha.com to calculate the deci- mal values if needed.