Just leave them be. They are there for multitasking purposes.
Apple has the best implementation of multitasking and ram-handling.
Any apps not used for some time fall in an idle-state where they no longer use any resources like battery, memory or CPU power.
You can force close them by swiping them up but by opening that app again that causes iOS to reload all information back to memory which in itself leads to extensive battery use during that period.
Keeping apps in multitask is faster, battery friendly and more convenient for you as user.
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