Неравенства 1) 4-2x< 9 2) x^2-4x-21> 0 3) 1-16x^2> 0 4) 7x-21x^2< 0

1) 4-2x<9 => -2x<9-4 => x>2.5
2) x^2-4x-21>0 => D=(-4)^2-4*(-21)=16+84=100=10; x1 = (4+10)/2=7; x2=(4-10)/2=-3; ответ: (-3;7)
3) 1-16x^2>0 => - 16x^2>-1 => 4x<1 => x< 0.25
4) 7x-21x^2<0. / x=> 7-21x<0 => - 21x<-7 => x<1/3