во втором действии ответ 1 1/4
3) 1 1/4*1 3/5=5/4*8/5=2
1.
(3m - 20) : 5 = 50,
(3m-20) = 50*5
(3m-20)=250
3m=250+20
3m=270
m=270:3
m=90,
2.
480:(13-t)+20=100,
480:(13-t)=100-20=80,
13-t=480:80=6,
t=13-6,
t=7
Сos²2x-cos2x=0
cos2x(cos2x-1)=0
cos2x=0
2x=π/2+πk
x=π/4+πk/2,k∈z
cos2x=1
2x=π+2πn
x=π/2+πn,n∈z