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Abel's theorem in problems and solutions - School of Mathematics

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The complex numbers 59<br />

219. Prove that two vectors are equal if <strong>and</strong> only if their coord<strong>in</strong>ates<br />

are equal.<br />

The set <strong>of</strong> equal vectors is considered to be a unique vector, characterized<br />

by its coord<strong>in</strong>ates, which is called a free vector. Putt<strong>in</strong>g <strong>in</strong>to<br />

correspondence every complex number with the free vector hav<strong>in</strong>g<br />

coord<strong>in</strong>ates we obta<strong>in</strong> the second geometrical representation <strong>of</strong> the<br />

complex numbers.<br />

220. Let <strong>and</strong> be the free vectors correspond<strong>in</strong>g to the complex<br />

numbers <strong>and</strong> Prove that if <strong>and</strong> only if<br />

where the vectors are added accord<strong>in</strong>g to the parallelogram rule.<br />

221. Prove the follow<strong>in</strong>g relation between the two geometrical representations<br />

<strong>of</strong> the complex numbers: if are the complex<br />

numbers correspond<strong>in</strong>g to the po<strong>in</strong>ts A, B <strong>and</strong> to vector then<br />

By def<strong>in</strong>ition, two equal vectors have equal lengths. This length is<br />

additionally assumed to be the length <strong>of</strong> the free vector correspond<strong>in</strong>g to<br />

a given set <strong>of</strong> equal vectors.<br />

DEFINITION. One calls the modulus <strong>of</strong> the complex number (denoted<br />

by the length <strong>of</strong> the correspond<strong>in</strong>g free vector 4 .<br />

222. Let Prove that<br />

where is the conjugate <strong>of</strong><br />

223. Prove the <strong>in</strong>equalities:<br />

a)<br />

b)<br />

where are arbitrary complex numbers. In which case does the equality<br />

hold?<br />

224. Prove by means <strong>of</strong> the complex numbers that <strong>in</strong> any parallelogram<br />

the sum <strong>of</strong> the squares <strong>of</strong> the lengths <strong>of</strong> the diagonals is equal to<br />

the sum <strong>of</strong> the squares <strong>of</strong> the lengths <strong>of</strong> all the sides.<br />

4 For real numbers (as a particular case <strong>of</strong> complex numbers) the notion <strong>of</strong> modulus<br />

<strong>in</strong>troduced here co<strong>in</strong>cides with the notion <strong>of</strong> absolute value. Indeed, to the real number<br />

there corresponds the vector with coord<strong>in</strong>ates parallel to the <strong>and</strong><br />

its length is equal to the absolute value <strong>of</strong> the number

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