I am not certain what you are asking. To have an 'inside diameter" you have to have one circle insides another (or a circle insidesomething) but see no mention of a circle. And you certainly have no "36 inch diameter" in a 3 and half inch region! It is possible that you want to make a cut from one side of the timber that will exit the other side of the timber at a distance of 36 inches. In that case, looking from the side, there is a right triangle with one leg, the end of the timber, of length 3.5 inches, another leg, the length to the end of the cut, of 36 inches. That is a right triangle in which we are given the "opposite side" and the "near side". The angle is given by .