D cм рис.
1 ≤ x ≤ 2
0 ≤ y ≤ (4-x)/2
m= ∫^(2)_(1) [b]([/b] ∫ ^((4-x)/2)_(0)3x dy[b])[/b]dx=
=∫^(2)_(1) 3x [b]([/b] y)|^((4-x)/2)_(0) dx=
=(3/2)∫^(2)_(1) (4x-x^2)dx=
=(3/2)*(2x^2-(x^3/3))|^(2)_(1)=
=(3/2)*(2*(2^2-1)-(1/3)*(8-1))=
считайте...