Question

Решить уравнения : cos2x-5cosx-2=0 1-cos8x=sin 4x sin^2x+4sinx*cosx+3cos^2x=0 cos4x-sin4x=-1/2 xsin^2x+sin^2 3x+sin^2 4x+sin^2 5x=2

Answer 1

1)cos2x-5cosx-2=0;⇒
2cos²x-1-5cosx-2=0;⇒cosx=y;-1≤y≤1;⇒
2y²-5y-3=0;
y₁,₂=(5⁺₋√(25+24))/4=(5⁺₋7)/4;
y₁=(5+7)/4=3;⇒y₁>1⇒нет решения;
y₂=(5-7)/4=-1/2;⇒
cosx=-1/2;⇒x=⁺₋2π/3+2kπ;k∈Z.
2)1-cos8x=sin4x;⇒
sin²4x+cos²4x-cos²4x+sin²4x=sin4x;⇒
2sin²4x-sin4x=0;⇒
sin4x(2sin4x-1)=0;⇒
sin4x=0⇒4x=nπ;k∈Z;⇒x=nπ/4;n∈Z;
2sin4x-1=0;⇒
sin4x=1/2;
4x=(-1)ⁿ·π/6+nπ;n∈Z.
3)sin²x+4sinxcosx+3cos²x=0;⇒cos²x≠0 делим на cos²x:
tg²x+4tgx+3=0;tgx=y;⇒
y²+4y+3=0;
y₁,₂=-2⁺₋√(4-3)=-2⁺₋1;
y₁=-1;⇒
tgx=-1;⇒x=-π/4+nπ;n∈Z;
y₂=-3;⇒x=arctg(-3)+nπ;n∈Z.
4)cos4x-sin4x=-1/2;⇒cos4x=sin4x-1/2;⇒
cos²4x=sin²4x-2/2·sin4x+1/4;⇒
1-sin²4x-sin²4x+sin4x-1/4=0⇒
-2sin²4x+sin4x+3/4=0;⇒
sin4x=y;-1≤y≤1;
2y²-y-3/4=0;
y₁,₂=(1⁺₋√(1+6))/4=(1⁺₋√7)/4;
y₁=(1+√7)/4=(1+2.646)/4=0.9115;
sin4x=0.9115;⇒4x=(-1)ⁿarcsin(0.9115)+2nπ;n∈Z;
x=(-1)ⁿ(arcsin(0.9115))/4+nπ/2;n∈Z;
y₂=(1-2.646)/4=-0.4115;
4x=(-1)ⁿarcsin(-0.4115)+2nπ;n∈Z
x=[(-1)ⁿarcsin(-0.4115)+2nπ]/4.