Why are airplane wings swept back?

Related questions

1. Why are airplane wings tipped up at the end?
2. How long does it take to refuel an a320?
3. What is a320 sharklet?
4. Did a 747 ever crash?
5. How do sharklets reduce fuel consumption?
6. How do winglets improve performance?
7. Why do some planes not have winglets?
8. What is the difference between a sharklet and a winglet?
9. What does Sharklets mean?
10. Do winglets reduce turbulence?

Tags

1. #supersonic
2. #tested
3. #worse
4. #slower
5. #variable
6. #sound
7. #friction
8. #achieve
9. #speed
10. #fighter
11. #planes
12. #results
13. #reduce
14. #moving
15. #airliners
16. #thinner
17. #swept
18. #sweep
19. #wings
20. #relatively