`a+(-a)=(-a)+a` is called…………….

This is called

The contraction of heart is called ........... while the relaxation of heart is called …………… .

A series resonance circuit is called an ……………..and a……………………is called ………………

Area of retina with only cones is called ………….. while area of retina with no photoreceptors is called ……………. .