State the limits for B if given that B is an obtuse angle and
a) tan2B is negative
b) tan4B is positive
c) tan2B is negative or tan 4B is positive
From the given it must be true that . Thus any answer you find must be in that range.
If then or , which is in that range.
If then it could be or , which is NOT in that range.
So we must take another course.
Suppose that or then .
Solving we get , which is in the range.