If we want to find out the the equation of the angle bisector of one of the angles of a triangle, how do we find out which sign(+/) to take? Sir had told this in class but I can't remember now.

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2 Answers
It depends on whether the angle formed between them is acute or obtuse and signs of the coefficients
Let lines be
ax + by + c = 0 and a'x + b'y + c' = 0
Firstly, write ax + by + c = 0 and a'x + b'y + c' = 0 such that constant terms are positive.
If aa' + bb' > 0, then the equation of the acute angle bisector is with + sign and obtuse is with  sign
If aa' + bb' < 0, then the equation of the acute angle bisector is with  sign and obtuse is with + sign
There was sumthing sort of this also....by making the signs of the constants of the 2 straight lines equal and then taking the + sign in the equation.....
ax+by+c/(√a^2+b^2) = a'x+b'y+c'/(√a'^2+b'^2)