For all geometry questions, you don't necessarily need to consider B as 90 degree, unless otherwise it is specifically mentioned in the question.
You can assume any one of A,B,C as 90 degree and solve the problem. Only requirement is that the side opposite to right angle is the hypotenuse.
If B is 90 degree, then a^2 + c^2 = b^2
If A is 90 degree, then b^2 + c^2= a^2
If C is 90 degree, then a^2 + b^2 =c^2
where a,b, and c are sides opposite to angle A,B, and C respectively.
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