Here we go -

a root — i   ($i=\sqrt{-1}$)
a symbol — pi
a division — pri
a landing — pier
dullness — tripe
brightness — esprit
a counter — riposte
a liar — imposter
a lie — misreport
commanders — imperators
substitutes — temporaries
instruments — polarimeters or vaporimeters

Here we go -

a root — i   ($i=\sqrt{-1}$)
a symbol — pi
a division — rip
a landing — pier
dullness — tripe
brightness — esprit
a counter — riposte
a liar — imposter
a lie — misreport
commanders — imperators
substitutes — temporaries
instruments — polarimeters or vaporimeters

Here we go -

a root — i   ($i=\sqrt{-1}$)
a symbol — pi
a division — pie   (this doesn't seem right to me)
a landing — pier
dullness — tripe
brightness — esprit
a counter — riposte
a liar — imposter
a lie — misreport
commanders — imperators
substitutes — temporaries
instruments — polarimeters or vaporimeters

Here we go -

a root — i   ($i=\sqrt{-1}$)
a symbol — pi
a division — pri
a landing — pier
dullness — tripe
brightness — esprit
a counter — riposte
a liar — imposter
a lie — misreport
commanders — imperators
substitutes — temporaries
instruments — polarimeters or vaporimeters