cos7+cos79+cos151+cos223+cos295=
ceo dan pokusavam da resim ovo pa ako neko zna da mi pomogne unapred hvala
ceo dan pokusavam da resim ovo pa ako neko zna da mi pomogne unapred hvala
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cos7+cos79+cos151+cos223+cos295=
ceo dan pokusavam da resim ovo pa ako neko zna da mi pomogne unapred hvala
ako se racuna preko digitrona ispadne 0 ali ja nikako postupno ne mogu da dodjem
znaci ne mozes?
cos7 + cos79 + cos151 + cos223 + cos295
Radimo sve postupno zbog preglednosti:
cos79 = cos(72+7) = cos72*cos7 - sin72*sin7 =
= cos7*cos72 - 2*sin36*cos36*sin7
cos151 = cos(180 - 36 + 7) = - cos(-36 + 7) =
= -(cos(-36)*cos7 - sin(-36)*sin7) = -cos7*cos36 - sin36*sin7
cos223 = cos(180 + 36 + 7) = - cos(36 + 7) =
= -(cos36*cos7 - sin36*sin7) = -cos7*cos36 + sin36*sin7
cos295 = cos(-72 + 7) = cos(-72)*cos7 - sin(-72)*sin7 =
= cos72*cos7 + sin72*sin7 =
= cos7*cos72 + 2*sin36*cos36*sin7
Sada sve saberemo i dobijamo:
cos7 + cos7*cos72 - 2*sin36*cos36*sin7 - cos7*cos36 - sin36*sin7 - cos7*cos36 +sin36*sin7 + cos7*cos72 + 2*sin36*cos36*sin7 =
= cos7 + 2*cos7*cos72 - 2*cos7*cos36 =
= cos7 * (1 + 2*cos72 - 2*cos36) =
= cos7 * (1 + 2*(2*cos[SUP]2[/SUP]36 - 1) - 2*cos36) =
= cos7 * (4*cos[SUP]2[/SUP]36 - 2*cos36 - 1) =
= cos7 * (cos36 - (1+5[SUP]0.5[/SUP])/4) * (cos36 + (1-5[SUP]0.5[/SUP])/4) = 0
Zato sto je druga zagrada jednaka nuli...
Nije tesko dokazati da je cos36 = cos(pi/5) = (1+5[SUP]0.5[/SUP])/4)
cos7 + cos79 + cos151 + cos223 + cos295
Radimo sve postupno zbog preglednosti:
cos79 = cos(72+7) = cos72*cos7 - sin72*sin7 =
= cos7*cos72 - 2*sin36*cos36*sin7
cos151 = cos(180 - 36 + 7) = - cos(-36 + 7) =
= -(cos(-36)*cos7 - sin(-36)*sin7) = -cos7*cos36 - sin36*sin7
cos223 = cos(180 + 36 + 7) = - cos(36 + 7) =
= -(cos36*cos7 - sin36*sin7) = -cos7*cos36 + sin36*sin7
cos295 = cos(-72 + 7) = cos(-72)*cos7 - sin(-72)*sin7 =
= cos72*cos7 + sin72*sin7 =
= cos7*cos72 + 2*sin36*cos36*sin7
Sada sve saberemo i dobijamo:
cos7 + cos7*cos72 - 2*sin36*cos36*sin7 - cos7*cos36 - sin36*sin7 - cos7*cos36 +sin36*sin7 + cos7*cos72 + 2*sin36*cos36*sin7 =
= cos7 + 2*cos7*cos72 - 2*cos7*cos36 =
= cos7 * (1 + 2*cos72 - 2*cos36) =
= cos7 * (1 + 2*(2*cos[SUP]2[/SUP]36 - 1) - 2*cos36) =
= cos7 * (4*cos[SUP]2[/SUP]36 - 2*cos36 - 1) =
= cos7 * (cos36 - (1+5[SUP]0.5[/SUP])/4) * (cos36 - (1-5[SUP]0.5[/SUP])/4) = 0
Zato sto je druga zagrada jednaka nuli...
Nije tesko dokazati da je cos36 = cos(pi/5) = (1+5[SUP]0.5[/SUP])/4)
Ako je problem i resavanje jednacine treceg stepena, evo i to:
4*x[SUP]3[/SUP] + 2*x[SUP]2[/SUP] - 3*x - 1 =
= 4*x[SUP]3[/SUP] -3*x - x + x + 2*x[SUP]2[/SUP] - 1 - 1 + 1 =
= 4*x[SUP]3[/SUP] - 4*x +2*x[SUP]2[/SUP] - 2 + (x + 1) =
= 4*x * (x[SUP]2[/SUP] - 1) + 2 * (x[SUP]2[/SUP] - 1) + (x + 1) =
= (x[SUP]2[/SUP] - 1) * (4*x + 2) + (x + 1) =
= (x + 1) * (x - 1) * (4*x + 2) + (x + 1) =
= (x + 1) * ( (x - 1)*((4*x + 2) + 1) =
= (x + 1) * (4*x[SUP]2[/SUP] - 2*x - 1) =
Odave sledi da je prvo resenje x = -1, a druga dva se dobijaju resavajuci ovu kvadratnu jednacinu...
A sta da je bio negativan cos?
Ok kapiram....