Present value, annuities and annuity due: CFA Level I TVM
The present value of an amount due in n periods is the amount times 1 / (1 + i)^n, where i is the rate per period. The value of a series of cash flows is the sum of each one discounted for its own date. An annuity due pays at the start of each period, so it is worth the ordinary annuity times (1 + i).
One amount
Treasury's auction rules (31 CFR Part 356, Appendix B) write the present value of 1 due at the end of n periods as v^n = 1 / (1 + i/2)^n, where i is the annual yield with semiannual compounding and n counts half-years. The general form is PV = FV / (1 + rate per period)^(number of periods).
Example: a zero-coupon bond pays 100 in 5 years at a yield of 4.00% with semiannual compounding. Rate per period = 2%, periods = 10. PV = 100 / 1.02^10 = 82.03. Using 4% for 5 periods instead would give 82.19, which is wrong.
Annuities and annuity due
The same Treasury appendix gives the present value of 1 per period for n periods as a(n) = (1 - v^n) / i, where i is the rate per period. This is v + v^2 + ... + v^n, an ordinary annuity with payments at the end of each period.
Example: 1,000 at the end of each of the next 3 years at 5%. v^3 = 1 / 1.05^3 = 0.863838. a(3) = (1 - 0.863838) / 0.05 = 2.72325. PV = 2,723.25.
If the payments come at the start of each year, the first one is paid today and each is one period earlier, so PV = 2,723.25 x 1.05 = 2,859.41. Equivalently, 1,000 + 1,000 / 1.05 + 1,000 / 1.05^2 = 1,000 + 952.38 + 907.03 = 2,859.41.
Uneven cash flows
When the amounts differ, discount each one for its own date and add. A share is worth the sum of its expected future cash flows discounted at a rate that reflects their risk, which is the starting point of every dividend discount model.
Example: dividends of 3 at the end of years 1 and 2 and a final payment of 50 at the end of year 3, at 10%. PV = 3 / 1.10 + 3 / 1.10^2 + 50 / 1.10^3 = 2.7273 + 2.4793 + 37.5657 = 42.77.
Where the exam catches people
Believing a semiannual-pay yield can be applied per year: divide the annual rate by 2 and double the number of periods.
Believing an annuity due is discounted the same way as an ordinary annuity: each payment is one period earlier, so multiply the ordinary annuity value by (1 + i).
Believing the first payment of an annuity due is discounted for one year: it is paid today, so its discount factor is 1.
Believing uneven cash flows can be valued with one annuity factor: each amount is discounted separately for its own date.