Descartes does not use the famous formulaion of the cognito - 'I think, therfore I am' - in the meditations.

Descartes does not use the famous formulaion of the cognito - 'I think, therfore I am' - in the meditations. Explain why he doesnt use that formulation in the Meditations, how he thinks that the formulation that he does use is known, and why he thinks that that formuation -'I am, I exist- "whenever it is uttered by me, or conceived in my mind, is necessarily true: