Х между примерами это умножение
18) f '(x) = (-2)' * ctgx - (-2)* (ctgx)' = 0 * ctgx + 2 * (- 1/Sin²x) = - 2/Sin²x
19) f ' (x) = (Sin15x)' = Cos15x * (15x)' = 15Cos15x
20) f '(x) = [Cos(π/4 - 12x)] ' = - Sin(π/4 - 12x) * (π/4 - 12x)' = 12Sin(π/4 - 12x)
4sinπ/6+3tg²π/4+2cosπ/3=2*1/2 +3*1²+2*1/2=1+3+1=5
22.б=0,4m
Г=0,6х
23.б=...=2x/10y=0,2xy
Г=...6m/4n=1,5mn
24.б=...=5(2m-n)/5n=2m-n/n
Г=...=2(ab-c)/2(ac-b)=ab-c/ac-b
25.б=...=x/x(y-1)=1/y-1
Г=...=аb(a+1)/ab=a+1
(...)-это данная дробь