Recall Exercise 6 of Section 1.4 Consider the following definition of the
“⊳” symbol. Definition. Let x and y be integers. Write x ⊲ y if jx + ky = lk for some integer k.
Give a direct proof of the following statement.
If a ⊲ b and c ⊲ d, then a + c ⊲ b + d.