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In[]:=

Manipulate[Plot[a^2*vx+(1-a)^2*vy+2*a*(1-a)*cxy,{a,0,1},AxesOrigin{0,0},AxesLabel{a,"Var(aX + (1-a)Y)"}],{{vx,0.5,"Var(X)"},0,1},{{vy,0.5,"Var(Y)"},0,1},{{cxy,0,"Cov(X,Y)"},-1,1}]

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