COVX medical equipment

Original formula = CoV (3x, x)+CoV (3x, 4y)+CoV (-2y, x)+CoV (-2y, 4y).

=3D(X)+ 12COV(X，Y)-2COV(X，Y)-8D(Y)

=3D(X)- 10COV(X，Y)-8D(Y)

Many attributes are applied: COV(X, Y)=COV(Y, x).

COV(aX，bY)=abCOV(X，Y)

Cov (x, a) = 0, and a is constant.