A negative number added to a positive number can sum up to either a positive number, negative number or zero, depending on the size of the two numbers added together. When a positive number is added to a negative number of equal size, they will essentially cancel out each other and sum to zero.
If a large negative number is added to a small positive number, such as -10 and 1, the answer will be a negative number, which is -9 in this case. Similarly, a large positive number added to a small negative number will result in a positive number. This is unlike multiplication in which a negative number multiplied by a positive number will always result in a negative number.