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单词 AlgebraicSinesAndCosines
释义

algebraic sines and cosines


For any rational numberPlanetmathPlanetmathPlanetmath r, the sine and the cosine of the number rπ are algebraic numbersMathworldPlanetmath.

Proof.  According to the http://planetmath.org/node/11664parent entry, sinnφ and cosnφ can be expressed as polynomialsPlanetmathPlanetmath with integer coefficients of sinφ or cosφ, respectively, when n is an integer.  Thus we can write

sinnφ=P(sinφ),cosnφ=Q(cosφ),

where  P(x),Q(x)[x].  If  r=mn  where m,n are integers and n0,  we have

P(sinrπ)=sinnrπ=sinmπ= 0,Q(cosrπ)=cosnrπ=cosmπ=±1,

i.e. both sinrπ and cosrπ satisfy an algebraic equation.  Q.E.D.

For example,

cos7φ= 64cos7φ-112cos5φ+56cos3φ-7cosφ,

whence we have the identity

64cos7π7-112cos5π7+56cos3π7-7cosπ7+1= 0,

and therefore cosπ7 is algebraic over .

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更新时间:2025/5/4 10:29:49