1)i have connected 3v dc supply ldr ant led in series

2)i have connected 3v dc supply with potential divider circuit by resistor and ldr and connected it to led

the second works good.does first one works

Define "works"

once it darken led off and once it darken led glow is this right way to connect

Hardly an Arduino related question, but;

If #2 works than #1 should too. Just make sure the LDR will handle the current through the LED.