`f(5)-f(4)=a.5^3+b.5^2+c.5+d-(a.4^3+b.4^2+c.4+d)`
`=125a+25b+5c+d-64a-16b-4c-d`
`=61a+9b+c=2019`
`f(7)-f(2)=a.7^3+b.7^2+c.7+d-(a.2^3+b.2^2+c.2+d)`
`=343a+49b+7c+d-8a-4b-2c-d`
`=335a+45b+5c5^2+c.5+d-(a.4^3+b.4^2+c.4+d)`
`=343a+49b+7c+d-8a-4b-2c-d`
`=335a+45b+5c`
`=5(61a+9b+c)+30a`
`=5.2019+30a`
`=5(6a+2019)`
`a` dương
`⇒5(6a+2019)>0`
mà `5(6a+2019)` chia hết cho `5`
⇒ `f(7) -f(2)` là hợp số