Comment on Activity 7

Since `costext[(]x text[)]=sin(pi/2-x),`

`d/(dx)costext[(]x text[)]=d/(dx)sin(pi/2-x).`

Using the Chain Rule, we may rewrite the right-hand side:

`d/(dx)costext[(]x text[)]` `=cos(pi/2-x)d/(dx)(pi/2-x)`
  `=-cos(pi/2-x).`

Since `cos(pi/2-x)=sintext[(]x text[)],`

`d/(dx)costext[(]x text[)]=-sintext[(]x text[)].`