TAILIEUCHUNG - Handbook of mathematics for engineers and scienteists part 8

If three sides of a triangle are congruent to the corresponding sides of another triangle, then the triangles are congruent. Triangles are said to be similar if their corresponding angles are equal and their corre-sponding sides are proportional. | . Some Mathematical Methods 17 Remark. The first step the formulation of an original hypothesis is the most difficult part of the method of mathematical induction. This step is often omitted from the method. . Proof by Counterexample A counterexample is an example which is used to prove that a statement proposition is false. Counterexamples play an important role in mathematics. Whereas a complicated proof may be the only way to demonstrate the validity of a particular theorem a single counter example is all that is need to refute the validity of a proposed theorem. in general the scheme of a proof by counterexample is as follows 1. Given a proposition all elements a that belong to a set A also belong to a set possess a property B. 2. Refutation of the proposition one specifies an element a counterexample that belongs to A but does not belong to B. Example. Proposition Numbers in the form 22 1 where n is a positive integer were once thought to be prime. These numbers are prime for n 1 2 3 4. But for n 5 we have a counterexample since 225 1 4294967297 641 X 6700417 it is a composite number. Conclusion When faced with a number in the form 22 1 we are not allowed to assume it is either prime or composite unless we know for sure for some other reason. . Method of Undetermined Coefficients The method of undetermined coefficients is employed to find coefficients in expressions such as formulas series expansions solutions to mathematical equations the form of which is either known in advance or assigned based on intuitive judgment. Example. The fractional function x 2 x x2 - 1 whose denominator can be rewritten in the factored form x x 1 x - 1 can be represented as the sum of partial fractions A B C x x 1 x - 1 where A B C are undetermined coefficients whose values are to be found. Equating with multiplying by x x2 - 1 and rearranging one obtains A B C x2 -B C - 1 x - A - 2 0. For this equation to be valid for any x the .

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