Đáp án:
Giải thích các bước giải:
`f(2)=a.2^3+b.2^2+c.2+d=8a+4b+2c+d`
`f(4)=a.4^3+b.4^2+c.4+d=64a+16b+4c+d`
`f(5)=a.5^3+b.5^2+c.5+d=125a+25b+5c+d`
`f(7)=a.7^3+b.7^2+c.7+d=343a+49b+7c+d`
Ta có
`f(5)-f(4)=2020`
`=>(125a+25b+5c+d)-(64a+16b+4c+d)=2020`
`=>61a+9b+c=2020`
Xét
`f(7)-f(2)=(343a+49b+7c+d)-(8a+4b+2c+d)`
`=335a+45b+5c`
`=30a+5(61a+9b+c)`
`=30a+5.2020`
`=10(3a+1010)` $\vdots$ `10` vì `a∈Z`
`=>đ.p.c.m`
