3.
cos(4x+π/4) = -√2/2 ;
4x+π/4 = ±(π -π/4) +2π*n ,n∈Z.
x = -π/16 ±3π/16 + π*n/2 ,n∈Z.
x₁ = -π/4 +π*n/2 ,n∈Z. * * * -3π/4 ; -π/4 ;3π/4 * * *
x₂ = π/8 +π*n/2 ,n∈Z. * * * -7π/8 ; -3π/8 ;π/8 * * *
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5. arccos(2x² -1) =arccos(3x+1);
ОДЗ : { |2x² -1| ≤1 ; 3x+1 ≤1.⇔ -1 ≤ x ≤ 0
2x² -1 =3x+1 ;
2x² -3x -2 =0 ;
[ x = -1/2 ; x= 4.
ответ : -1/2.