∫−ππ∣π−∣x∣∣dx
Let f(x)=∣π−∣x∣∣
Replace x by −x.
f(−x)=∣π−∣−x∣∣⇒f(−x)=∣π−∣x∣∣
Since f(x)=f(−x)⇒f(x) is an even function.
Now by the property of definite integration for even function.
∫−ππ∣π−∣x∣∣dx=2∫0π∣π−∣x∣∣dx
=2∫0π∣π−x∣dx(∵x>0)
=2∫0π(π−x)dx(∵π−x>0∀x∈(0,π))
=2(πx−2x2)0π
=2(π2−2π2−0−0)=π2