These materials are a part of a series of studies sponsored by the Cambridge Conference on School Mathematics which reflects the ideas of CCSM regarding the goals and objectives for school mathematics K-12. Feasibility Studies 9-13 contain a wide range of topics. The following are the titles and brief descriptions of these studies. Number 9--"Streams of Ideas on Checks, Approximations, and Order of Magnitude Calculations." This paper suggests that students should be made aware of major sources of errors in calculations so that they can concentrate on learning mathematical concepts. Number 10--"Complex Numbers Leading to Trigonometry." This study introduces the reader to Trigonometry by the use of complex numbers. Number 11--"The Use of Negative Digits in Arithmetic." Negative integers are introduced after the four fundamental operations have been mastered. The notation 3 is used for negative three. Number 12--"Use of the Shift Theorem in Differential Equations." This study describes the use of the Shift Theorem to solve certain types of differential equations. Number 13--"Topology in 10th Grade and After." A list of topological concepts which are relevant to high school mathematics is given in this study. [Not available in hardcopy due to marginal legibility of original document.] (FL)