Let {π‘π}ππ=1 be an K-spanning set for π and {ππ}ππ=1 be a π-spanning set for π.
Then
{π‘πππ:πββπ,πββπ}is an K-spanning set for π, since any π£ βπ may be expressed as an π-linear combination of ππ's and the coΓ«fficients may then be expressed as linear combinations of π‘π's.