Do you decline a multiplier in reading a mathematical formula in Russian? How do you read "Порядок определителя равен $2n$"? Is it "двум эн" or is it "два эн"?
And in a sum, do you read $c = a_5 + a_6$ as "це равно а пятому плюс а шестому"? Or does the plus sign interfere with the declension in some way?

Since Grigory M won't be posting an answer, I will post the answer I've received here, for future reference.
For the question about $2n$, it seems that the multiplying numeral $2$ can either be left invariable as "два" or put in the dative as "двум," with the latter option considered more correct by some people who have an opinion on the matter.
For $c = a_5 + a_6$, there are two options. Either retain the indices as adjectives by saying "це равно сумме а пятого и а шестого" ("$c$ is equal to the sum of $a$ the fifth and $a$ the sixth"), or, less elegantly, convert the indices to undeclined numerals as in "це равно а пять плюс а шесть" ("$c$ is equal to $a$ five plus $a$ six.") The reading "це равно а пятому плюс а шестому" ("$c$ is equal to $a$ the fifth [dat.] plus $a$ the sixth [dat.]") proposed in the OP is apparently not possible.
