Answer:
Because of the range of the sine function.
Step-by-step explanation:
The reason why the value of the sine ratio for an acute angle
of a right triangle must always be a positive value less
than 1 has to do with the the range of the sine function.
The range of the sine function is
But in the first quadrant all y-values are positive and acute angles are less than 90°.
Therefore the range will now be 0≤y<1 for acute angles.