Derive Ampere's circuital law

In this video the derivation of Ampere's circuital law has been derived from the relation of curl of magnetic field.

[mebalishamynsong]

How do we know that we have to use with cos

[bigadaraees]

thanks a lot
but where's that figure

[vibhavshivadeker]

Bro, this is not derivation😂😂
U just changed some mathematical operators there