# Math logic between two concepts.

##### Full Member
given an equation like X^2+Y^2=R^2 afterwards in the video, the teacher said we have a case that y=0
then he said that case must satisfy also X^2+Y^2=R^2 , then he assigns y=0 into the equation and said we have X^2=R^2 .. my question is how really we can assign to the equation? isn't the equation X^2+Y^2=R^2 specified for specific case?

#### Subhotosh Khan

##### Super Moderator
Staff member
given an equation like X^2+Y^2=R^2 afterwards in the video, the teacher said we have a case that y=0
then he said that case must satisfy also X^2+Y^2=R^2 , then he assigns y=0 into the equation and said we have X^2=R^2 .. my question is how really we can assign to the equation? isn't the equation X^2+Y^2=R^2 specified for specific case?
Which video?