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D= -7+2=-5
a3=a1+2d
a3= -2+2(-5)=-2-10= -12
a4=a1+3d
a4= -2+3(-5)= -2-15= -17
a1+a2+a3+a4= -2-7-12-17= -38
или S4=((2a1+(n-1)d)/2)n= ((-4-15)/2)*4= -19*2= -38
sin2a+sin6a=2sin4a cos2a
cos2a+cos6a=2coa4a cos2a
(sin2a+sin6a)/(cos2a+cos6a)=(2sin4a cos2a)/(2coa4a cos2a)=sin4a/cos4a=tg4a
2х+7>3x+4
7-4>3x-2x
3>x
при х меньше или равно 3