A cross between `F_(1)` hybrid and its homozygous recessive parent is called

A cross between and F_1 hybrid and its homozygous recessive parent is called :

What is the cross between the F_(1) Progeny and the Homozygous recessive parent called ? How is it useful ?

What is the cross between the progeny of F_(1) and the homozygous recessive parent called? How is it useful?

What is the cross between the progeny of F_(1) and the homozygous recessive parent called? How is it useful?

The phenotypic ratio in a back cross between a trihybrid and homozygous recessive parent would be :-