Let x=et+e−t and y=et−e−t for all real t.
a. (i) Obtain an expression for dxdy in terms of t.
b. (ii) Show that the curve has no horizontal tangents.
c. (iii) Determine all real values of t at which the curve has a vertical tangent, and find the corresponding point(s).
d. (iv) Write the equation(s) of the vertical tangent line(s) found in part (iii).