Why I can't use acute trigonometric function in obtuse angles? I need help at understanding why acute trigonometric functions only work with right triangles. I would appreciate any help.

The trigonometric functions are defined in terms of ratios of sides of a right angled triangle. $\sin\theta$ is defined to be the ratio between the side opposite an angle $\theta$, and the hypotenuse of a right angled triangle. These ratios change when you change the right angle to an obtuse angle, which is why they no longer hold.
