summation vs summing up

summation and summing up both are nouns.

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

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

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

Other synonyms of summation include rundown.

summation (noun)	summing up (noun)
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)
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