We have a triangle:
the side opposite the angle Ф is 4/5,
The side adjacent the angle Ф is 3/5
1) we calculate the hypotenuse, by the Pythagoras Theorem:
h=√[(4/5)²+(3/5)²]=1
2) we calculate the csc of Ф
csc Ф= hypotenuse / opposite side
csc Ф=1/ (4/5)=5/4
Answer: e. 5/4
