When x = 0, t = a and when x = a, t = 0 Now
proceed as in P3 Proof of P5 Using P2, we have 
2
2
0
0
( )
( )
( )
a
a
a
a
f x dx
f x dx
f x dx
=
+
∫
∫
∫ Let
t = 2a – x in the second integral on the right hand side