Ответы и объяснения

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2011-12-16T11:06:31+04:00

ctg40*tg140=cos(79-34)

ctg40*tg140=cos45

cos40/sin40*sin140/cos140=sqrt2/2

cos40/sin40*cos40/sin40=sqrt2/2

ctg^2(40)=sqrt2/2

 

2011-12-16T11:58:30+04:00
cosalfa*cosbeta+sinalfa*sinbeta=cos(alfa-beta)cos79*cos34+sin79*sin34=cos(79-34)=cos45=sqrt(2)/2.  cosalfa*sinbeta=1/2*( sin(alfa-beta) + sin(alfa+beta)ctg40*tg140=cos40/sin40 * sin140/cos140= (cos40*sin140)/(sin40*cos140)==[1/2 * (sin(40-140)+sin(40+140))]/[1/2 * (sin(140-40)+sin(140+40)] =(sin(-100)+sin180)/(sin100+sin180)= - sin100/sin100= -1