U1 and U2 are the strain energies stored in a prismatic

U₁ and U₂ are the strain energies stored in a prismatic bar due to axial tensile forces P₁ and P₂, respectively. The strain energy U stored in the same bar due to combined action of P₁ and P₂ will be

u₁ =	1	.P₁ × δl
	2

=	1	× P₁ ×	P₁ × l
	2		AE

=	1	×	P₁²L
	2		AE

u₂ =	1		P₂²L
	2		AE

=	1		(P₁² + P₂²)L
	2		AE

u =	1		(P₁² + P₂² + 2P₁P₂)L
	2		AE

= u₁ + u₂ +	1		2P₁P₂L
	2		AE

∴ u > u₁ + u₂