U-substitution

Forum|Forum|9 years ago
May 23, 2017
4 replies
1848 views

hello all,

i am trying to get my head around integration by substitution.

in a simple case of the integrand (x-1)/2 dx I know the answer is (x^2/4)-(x/2).

if I try to do this by substitution, I get a bit lost.

ive gone as follows:

u= x-1

du=1*dx

dx=du

(u/2)du

(u^2)/4

(x-1)^2/4

(x^2-2x+1)/4

obviously this is not correct. Does anyone know what I'm doing wrong?

‌

Calculus_Derivatives

Best answer by AlanStevens

You have done it correctly! The apparent difference is because an indefinite integral is only defined to within an arbitrary constant. The two forms you show are the same to within an arbitrary constant.

Alan

A

AlanStevens

Answer

19-Tanzanite

You have done it correctly! The apparent difference is because an indefinite integral is only defined to within an arbitrary constant. The two forms you show are the same to within an arbitrary constant.

Alan

A

awibroe

Author

1-Visitor

‌Alan,

thanks for that, interesting!

so if I make the original integral limited between 2 and 4 I believe the end result ends up being 2. When doing this for the substitution method, the limits I believe become 1 and 3 noting that my I value is x-1. This produces an answer of 1 not 2?

andy