28у-42 -5у-20=6 у+10+5у
28у -5 у -6у-5у =42+20+10
12 у=72
у=6
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Cos 4x + cos 2x + √3*cos x = 0
![2cos \frac{4x+2x}{2}*cos \frac{4x-2x}{2} + \sqrt{3}*cos x = 0](https://tex.z-dn.net/?f=2cos+%5Cfrac%7B4x%2B2x%7D%7B2%7D%2Acos+%5Cfrac%7B4x-2x%7D%7B2%7D+%2B+%5Csqrt%7B3%7D%2Acos+x+%3D+0)
![2cos3x*cosx+ \sqrt{3}*cosx=0](https://tex.z-dn.net/?f=2cos3x%2Acosx%2B+%5Csqrt%7B3%7D%2Acosx%3D0+)
![cosx*(2cos3x+ \sqrt{3} )=0](https://tex.z-dn.net/?f=cosx%2A%282cos3x%2B+%5Csqrt%7B3%7D+%29%3D0)
1) cos x = 0;
x1 = pi/2 + pi*k
2) cos 3x = -√3/2
3x = +-11pi/6 + 2pi*n
x2 = +-11pi/18 + 2pi/3*n
f'(x)=(6x-x^3)'=-3x^2+6
-3x^2+6=0
3x^2=6
x^2=2
x1=sqrt2
x2=-sqrt2
x є (-оо; -sqrt2] U [sqrt2; +oo)