3+9+15+21+27+33+39+45+51+57+63+69+75=507
3sinx - cosx = 0
поделим обе части на cosx, при этом cosx не должно равняться 0
3sinx\cosx - cosx\cosx = 0
sinx\cosx = tgx
3tgx - 1 = 0
3tgx = 1
tgx = 1\3
x = arctg 1\3 + пи * n
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</span><span>
1)cos40</span>°соs20°= 1/2(cos(40°+20°)+cos(40°-20°) = 1/2(cos 60° + cos20°)= =1/2(1/2 +cos20°) = 1/4 + 1/2cos20°
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2) cos(</span>π/4+y)sin(π/4-y)= 1/2(sin (π/4+у+π/4-у)+sin(π/4+у-π74+у)) =
= 1/2(sin π/2+sin 2у)= 1/2(1+sin 2у) = 1/2 + 1/2sin2y
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3)cos(x+y)cos(x-y) = </span>1/2(cos(х+у+х-у)+cos(х+у-х+у) = 1/2(cos2х + cos2у)=
=1/2cos2x + 1/2cos2y
3^2x-19= 1/3^4
3^2x-19= 3^-4
основания равны,можна степени приравнять
2x-19=-4
2x=-4+19
2x=15
x=15/2
x=7,5