18) Find the center, foci, and vertices of the ellipse, and determine the length of the major and minor axes. Then sketch the graph. a) b) Center: Foci: Vertices: 19) Find the solutions of the system of equation. 20) The population of a certain city was 112,000 in 2006, and the observed doubling time for the population in 18 years. a) Find an exponential model for the population t years after 2006. b) Find an exponential model for the population t years after 2006. 21) Find the for the given system of equations.

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