If b is the mean proportional between a and c , thena2 −

If b is the mean proportional between a and c , then
a² − b² + c² = ?
a⁻² − b⁻² + c⁻²

Here b is the mean proportional between a and c .
∴ a : b : : b : c

⇒	a	=	b	⇒ b² = ac
	b		c

Now,	a² − b² + c²
	a⁻² − b⁻² + c⁻²

=	(a² − ac + c²) a²c²	= a²c²
	c² − ac + a²

(ac)² = (b²)² = b⁴