Koshi says that he knows that sin(x + y) = sin(x) + sin(y) because he has substituted in multiple values for x and
y, and they al work. he has tried x = 0° and y = 0°, but he says that usualy works, so he also tried x = 45° and
y = 180°, x = 90° and y = 270°, and several others. is koshi corect? explain your answer.