i am very confuse how my profs always use taylor expansion in physics which somehow doesn't follow the general equation of f(x) = f(a) + f'(a)(x-a) + 1/2! f''(a)(x-a)2 and so on... like for example, what is the taylor expansion of x - kx where k is small it was given as something like x - kx f'(x) + (1/2) k2 x2 f''(x) + ... is this taylor expansion? but there is no 'about which point, i.e, a=? ' i don't even understand how the first term x is gotten. f(a) = x?? please help thank you!