summing up vs summation

summing up and summation both are nouns.

Word	Noun	Adjective	Verb	Adverb
summing up	Yes	No	No	No
summation	Yes	No	No	No

As nouns, summing up and summation are synonyms defined as:

summing up and summation: a concluding summary (as in presenting a case before a law court)

Other synonyms of summing up include rundown.

summing up (noun)	summation (noun)
a concluding summary (as in presenting a case before a law court)	the arithmetic operation of summing; calculating the sum of two or more numbers
	a concluding summary (as in presenting a case before a law court)
	the final aggregate
	(physiology) the process whereby multiple stimuli can produce a response (in a muscle or nerve or other part) that one stimulus alone does not produce