How to simplify f(E)q(-E) + f(-E)q(E) with variable substitution

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koolmodee
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We have f(E)q(-E) + f(-E)q(E)

now change to x(E)=q(E) + f(E)/ (term)

and get -(f(E)f(-E))/(term)

I plugged the changed variable in, but didn't get the result, any hints how to proceed?

I got that from page 63, 64 equations 7.6, 7.7, 7.8 from this text: http://www.physics.ucsb.edu/~mark/ms-qft-DRAFT.pdf

thank you
 

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?? 7.6, 7.7, and 7.8 are nothing like what you give here! 7.6 involves an integral and "x" is a change of variable in the integral.
 
Just rearrange your variable substitution to isolate [itex]q(E)[/itex], and then determine [itex]q(-E)[/itex], and substitute into equation (7.6) and simplify.

No offense, but this should be pretty straightforward for someone who's studying QFT.