Question

If tan25°=a, then find the value of tan155° and tan115°

Answer 1

we are given that tan25°=a.
So,we have to find the value of 1)tan155° and 2)tan115°.
For 1) tan155°=tan(180-25)°= -tan25° = -a(since,180°<155°<90°, so it lies in 2nd quadrant where tangent of any angle is negative ,thus we put the (-)sign before it).
Now for 2) tan115° = tan(90+25)° = -tan25° = -a ( since 180°<115°<90°, so it lies in second quadrant where tangent of any angle is negative,thus we give the (-) sign before it)