Question

When tcos theta=mg holds good and when mg cos theta=t holds good in pendulum?

Answer 1

release a pendulum from rest at angle theta from eqm position. At this pt body has no motion . So balance forces along x and y axis and you get Tcos theta= mg.
Now release the bob. Now it undergoes circular motion and it again comes to rest on the other extreme position and again continued its circular motion. So the other extreme postn occurs for an instant when the body is undergoing circular motion and thus we balance forces along radial and tangential direction there and we write T= mg cos theta.