First, we need to remember that the cosine of an angle can be expressed as the sine of its complement angle. Therefore, we can rewrite the given expression as:

cos115 = sin(90-115) = sin(-25)

cos35 = sin(90-35) = sin(55)

cos65 = sin(90-65) = sin(25)

cos25 = sin(90-25) = sin(65)

Now, we can substitute these values into the original expression:

-sin(-25) + sin(55) + sin(25) + sin(65)

The negative sign in front of sin(-25) indicates that the sine function is an odd function, meaning sin(-x) = -sin(x). So, we can simplify the expression further:

-sin(-25) = -(-sin(25)) = sin(25)

Now the expression becomes:

sin(25) + sin(55) + sin(25) + sin(65) = 2sin(25) + sin(55) + sin(65)

Therefore, the simplified expression is 2sin(25) + sin(55) + sin(65).