計算第七題
證明:
積分(1/2 -->1) [f(x)/f(x+1/2)]dx
=積分(1/2 -->1) [f(x)/f(x-1/2)]dx
(令t=x-1/2)
=積分( 0 -->1/2) [f(t+1/2)/f(t)]dt
=積分( 0 -->1/2) [f(x+1/2)/f(x)]dx
因此
積分(0 -->1) [f(x)/f(x+1/2)]dx
=積分(0 -->1/2) [f(x)/f(x+1/2)]dx + 積分(1/2 -->1) [f(x)/f(x+1/2)]dx
=積分(0 -->1/2) [f(x)/f(x+1/2)]dx + 積分( 0 -->1/2) [f(x+1/2)/f(x)]dx
=積分(0 -->1/2) [f(x)/f(x+1/2)+f(x+1/2)/f(x)]dx
由算幾不等式
>=積分(0 -->1/2) [2]dx
=1,得證。