It is very important to note here that if x can be between
then 2x can be between
. When looking at the graph of cos(x)
to find values where cos takes the value 1, it is important to check all values between
On this interval, cos(2x)=1 when 2x is equal to , 0 and . This gives three solutions on the interval :
, and .
It is very common for students to lose marks on questions like this – they forget to extend the interval that caters for all possible solutions.