ax 1? +x 1+ax2? +x2-2{a[(x 1+x2)/2]? +[(x 1+x2)/2]}
={( 1/2)(2ax 1? +2ax2? )-2a[(x 1+x2)/2]? }+{(x 1+x2)-2 *[(x 1+x2)/2]}
=( 1/2)[2ax 1? +2ax2? -a*(x 1? +2x 1x2+x2? )]+0
=( 1/2)a[x 1? +x2? -2x 1x 2]=(a/2)(x 1-x2)? ;