Would the sum of 5.7 cm and 6.2 cm need to be rounded? why or why not?

No, the sum of 5.7 cm and 6.2 cm doesn't need to be rounded.

5.7 cm + 6.2 cm = 11.9 cm

Now,
Both 5.7 cm and 6.2 cm has 1 decimal place. So, their sum also has to be equal to 1 decimal place tooo. As you can see their sum also has 1 decimal place. So, you don't have to round it.

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.