1) 5cos2x -42sinx -13 =0 ;
5(1-2sin²x) - 42sinx -13 =0 ;
10sin²x) + 42sinx +8 =0 ;
5sin²x) + 21sinx +4 =0 ; * * * замена t =sinx ; |t| ≤1 . * * *
5t² +21t +4 = 0 ; * * * D =21² -4*5*4 =441- 80 =361 =19² * * *
t₁ =(-21-19)/2*5 = -4 * * * |t₁| = |-4| = 4> 0 . * * *
t₁ =(-21+19)/2*5 = -2/10 = -1/5.
[ sinx =- 4 ; sinx = -1/5.
sinx = -1/5 ;
x =(-1)^(n+1)arcsin(1/5) +πn , n∈Z.
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2) 11sin2x-6cos²x-4=0 ;
22sinxcosx -6cos²x -4(sin²x +cos²x) =0 ;
2sin²x -11sinx*cosx + 5cos²x =0 ;
2tq²x - 11tqx + 5 =0 ; * * * замена t =tqx ; * * *
2t² - 11t + 5 =0 ; * * * D =11² -4*2*5 =121- 40 =81 =9² * * *
[ t =(11- 9)/4=1/2; t =(11+9)/4=5.
tqx =1/2 ⇒ x =arctq(1/2) +πn ,n∈Z.
t =5 ⇒ x =arctq5 +πn ,n∈Z.