0
a
x
dx
x
a
x
+
−
∫
18 4
0
1
x
dx
−
∫
19 Show that 
0
0
( ) ( )
2
( )
a
a
f x g x dx
f x dx
=
∫
∫
, if f and g are defined as f(x) = f(a – x)
and g(x) + g(a – x) = 4
Choose the correct answer in Exercises 20 and 21 20