Transform and combine variables

E(a+bX)=a+bE(X), Var(a+bX)=b²Var(X), SD(a+bX)=|b|SD(X). Adding a constant changes location but not spread. For independent X and Y, Var(X+Y)=Var(X-Y)=Var(X)+Var(Y). Expectations add without requiring independence. In general, covariance terms are needed for variances of sums. A variance cannot be negative.