[ 62 7 ] 
But the excefs of above is the 
increment or fluxion (anfwering to the increment, 
or fluxion, x) anting by fubftituting b for a , /3 for a, 
and w for u. Moreover, with regard to the quanti- 
ties on the other fide of the equation, it is plain, fee- 
ing the difference of q[ and y" j^is indefinitely little 
in comparifon of their fum, that q may be fubfti- 
tuted in the room of &c. which being 
done, our equation will ftand thus : 
Flux, Qfl q 4 Rf &c. = q' ^ + r ' R &V* 
But q' 4 r r &c. reprefents (by the preceding 
notation) the fluxion of q f- r R &c. (or of $jq 
4 • Rr &c.) arifing by fubftituting a fory, making a 
alone variable, and calling off d. If, therefore, that 
fluxion be denoted by u, we fhall have flux. §?Jq 4 
R' r &c. = v, and confequently ^ q 4 R~ r &c. = u. 
But Qflq 4 R r&c. (by the fame notation) appears 
to be the fluxion of g^q' 4 Rr &c. (or of j ^. q 4 Rr 
&c.) arifing by fubftituting u for y , making u alone 
variable, and cafting off u\ Whence the following 
General Rule. 
Take the fluxion of the given expreflion (whofe fluent 
is required to be a maximum or minimum) making 
y alone variable ; and , having divided by y, let the 
quotient be denoted by Then take, again , the 
fluxion of the fame expreflion , makhig y alone va- 
riable , which divide by y and then this lafl quo - 
. Rent will be = v. 
4 L 2 
When 
