Скорость машины= 120/2 = 60 км в час
<span>скорость грузовика = 120/3 = 40 км в час</span>
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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
_________________ а-2) б-1) в-4)