(a) Which two (difficult) limits are important in the proof that the derivative of sin(x) is cos(x)? (b) Once we know the derivatives of sin(x) and cos(x), why don't we need to work a limit proof to show that the derivative of tan(x) is sec?(x) (with the proper domains)? (c) Once we know the tangent line at a point on a differentiable function, how to we find the normal line to the graph of that function? (d) For which values of n does the Power Rule -x" = nx-1 apply