Is it possible for the answer of sin49.5 to be negative?

If the 49.5 is in radians, then sin 49.5 ≈ −0.693 and so yes.

If the 49.5 is in degrees, then sin 49.5o ≈ 0.760

If the 49.5 is in gradians, then sin 49.5 ≈ 0.702

If the 49.5 is in some other angle measurement, then you'll have to decide as I only know Degrees, Radians and Gradians angle measures.

In Degrees, one full turn is 360o

In Radians, one full turn is 2π radians ≈ 6.283 radians

In Gradians, one full turn is 400 gradians.

Radians are most useful in calculus.

In fact you've used radians without realising it:

The length of an arc of angle θ of a circle of radius r is θr when θ is measured in radians; the length of an arc of a circle round one full turn (ie the circumference of a circle) is θr = 2πr since one full turn is 2π in radians.