dt = – dx When x = – a, t = a and when
x = 0, t = 0 Also x = – t INTEGRALS         343
Therefore
( )
a
a f x dx
−∫
 =
0
0
–
(– )
( )
a
a f
t dt
f x dx
+
∫
∫
=
0
0
(– )
( )
a
a
f
x dx
f x dx
+
∫
∫
       (by P0)