help geometry

P_1 P_2 + P_2 P_3 + P_3 P_4 + \dots + P_9 P_{10} + P_{10} P_1 is equal to 20.

In the problem, it wanted us to find $P_1 P_2 + P_2 P_3 + P_3 P_4 + \dots + P_9 P_{10} + P_{10} P_1$

Notice that this is literally just the perimeter of decagon $P_1 P_2 P_3 \dotsb P_{10}$

The length of one side of the decagon is

$\text{radius}/2 ( -1 + \sqrt {5})$

Multiplying this by 10 and we know the perimeter. We get

$10 (1/2) ( -1 +\sqrt 5) = 5 ( -1 + \sqrt 5) ≈ 6.18$

Thus, 6.18 is approximately our answer. 

Thanks! :)