# Is it true that the definite integral of x*f(x) = ((a+b)/2)*definite integral of f(x), where f(x) = a + b - x and the limits of the integration are a and b?

justaguide | College Teacher | (Level 2) Distinguished Educator

Posted on

It is given that f(x) = a + b - x

`int_a^bx*f(x)dx` = `int_a^bx*(a + b - x)dx`

= `int_a^b ax + bx - x^2 dx`

= a(b^2 - a^2)/2 + b(b^2 - a^2)/2 - (b^3 - a^3)/3

= ab^2/2 - a^3/2 + b^3/2 - ba^2/2 - b^3/3 + a^3/3 ...(1)

[(a + b)/2]`int_a^bf(x)dx` = [(a + b)/2]*`int_a^b(a + b - x)dx`

= [(a + b)/2]*[a(b - a) + b(b - a) - (b^2 - a^2)/2]

=   a(b^2 - a^2)/2 + b(b^2 - a^2)/2 -  (b^3 + a*b^2 - a^2*b - a^3)/4

= ab^2/2 - a^3/2 + b^3/2 - ba^2/2 - b^3/4 - ab^2/4 - a^2*b/4 - a^3/4 ...(2)

(1) and (2) are not equal.

It is not true that `int_a^b x*f(x)dx` = [(a + b)/2]*`int_a^bf(x)dx`

helga95 | (Level 1) Honors

Posted on

i'm sorry, but you are wrong again because my book got other response.

my book got that the equation is true but must be shown how.please, don't give me wrong answer.