The Problem May Lord... is related to problems such as expressing trigonometrical ratios of angle A/2 in terms of sinA.
There is always an ambiguity in such cases.
What I am trying to say is that knowing the value of sinA does not uniquely determine the value of sinA/2 and cosA/2 but only gives the magnitude or absolute value of sinA/2 and cosA/2.To obtain the trigonometrical ratios completely,of A/2,in terms of sinA we also need to find its sign [ +,-]. But,to determine the sign [ +,-] we need to know the quadrant in which the angle lies.
To find the ambiguities we can proceed d as follows:
sinA/2+cosA/2
=2(21sinA/2+21cosA/2)
=2sin(π/4+A/2)...(i)
(i) is positive ifA/2+π/4 lies between 2nπ and 2nπ+π
i.eA/2 lies between 2nπ−π/4 and 2nπ+3π/4
∴sinA/2+cosA/2 is positive if
A/2 lies between 2nπ−π/4 and 2nπ+3π/4
It is negative otherwise.
[i.e between 2nπ+3π/4 and 2nπ+7π/4 ]
In just similar way it can be shown that sinA/2−cosA/2 is positive
if A/2 lies between 2nπ+π/4 and 2nπ+5π/4.
It is negative otherwise.
[i.e between 2nπ−3π/4 and 2nπ+π/4 ]
It will be much better to understand if you have a diagram[I have a diagram but I don't know how to upload it here,sorry for that].Draw the renowned four quadrants then mark of π/4,3π/4,5π/4,7π/4.
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