Evaluate: `int_0^3[x]dx`

Explanation

`I=int_0^3[x]dx`

`I=int_0^1[x]dx`+`int_1^2[x]dx`+`int_2^3[x]dx`

`I=0int_0^1dx`+`1int_1^2dx`+`2int_2^3dx`

`I=1(2-1)`+`2(3-2)`

`I=1+2`

`I=3`