I reckon you can put this on your mistake book, when doing trig expansions the solution is
note you multiply the
and
terms instead of adding them. Eg. we are told that
, not
.
yashbb's mistake is very clear and is very common for newcomers to trig expansion, or people who have not done trig expansion for a long time, he understands the concept that
but his misstep is that
which comes from doing
, a pretty common mistake that can easily be corrected in the student's journey to mathematical proficiency.
A good mistake to make because you get to see where you went wrong early. Albeit mathematics is a learning experience.