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 The value of 
3
5
2
2
(
cos
tan
1)
x
x
x
x
dx
π
−π
+
+
+
∫
 is
(A) 0
(B) 2
(C) π
(D) 1
21