how can i enable double click touchpad on fedora 20 kde ?

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In KDE, this is very simple to do:

Method 1: Use Kickoff

1. Press the Kickoff button (left-bottom extreme, near the three colored dots).
2. Then, type Touchpad and click Enter.
3. Move over to Tapping.
4. Check Enable tapping if not already enabled.
5. Then, click on One finger, then click on Left click.
6. Hit apply, and then double tap should work.

Method 2: Using System Settings

1. Open System Settings, and find the search Touchpad or Input Devices.
2. If in Input Devices, click Touchpad on left panel.
3. Move over to Tapping.
4. Check Enable tapping if not already enabled.
5. Then, click on One finger, then click on Left click.
6. Hit apply, and then double tap should work.

Done! HTH, James