T= L/R = 4.5 X 10^-3/4 = 1.1 mS . F= 300 HZ, 1/F = 3.3mS. At 90% mark, pulse width = 3mS = 2.7 X T. So putting this into your formula. Gives e^- (TR/L) = e^ - (1/2.7) = e^ -.37 = 1/e^.37 = 1/1.44 = .69. So 1-.69 = .31, so the square bracketed bit becomes V/R X .31 .
Well that is the answer but I feel its wrong. We all know that the current is 63% of its final value when the time period = T, so if we say the exponential term is e^ (L/TR), then the numbers become:- e^-2.7 = 1/ e^2.7 = 1/14.8 = .067, 1- .067 = .932, so the square bracketed term becomes V/R X .932, which looks right.
What do you say?
Frank