f′(x)=limh→0f(x+h)−f(x)h
=
limh→01x+h−3sin(x+h)−1x+3sinxh
=
limh→01x+h−1x−(3sin(x+h)−3sinx)h
=
limh→01x+h−1xh+limh→03sin(x+h)−3sinxh
=
limh→0−1x(x+h)+3⋅limh→0sinx⋅cosh+cosx⋅sinh−sinxh
=
−1x2+3⋅limh→0sinx⋅(cosh−1)−cosx⋅sinhh
=
−1x2+3⋅sinx⋅limh→0cosh−1h−3⋅cosx⋅limh→0sinhh
=
−1x2−3⋅cosx