It depends on what value you are looking, but generally adding will result in a negative result(assuming you had a positve answer to begin with, and vice versa). so for instance if you have , now if you add you will basically have , so taking . now if instead of adding we decide to add we get so taking you'll find more information here Unit circle - Wikipedia, the free encyclopedia

Since we are given then .

now for the sine function, we are give then since sine is an odd function.

if we take we get now replacing by yields the desired result.