If x = a sec θ cos φ, y = b sec θ sin φ,

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If x = a sec θ cos φ, y = b sec θ sin φ, z = c tan θ, then the value of
x² +
y² -
z² is :
a² b² c²

x = a sec θ. cos φ; y = bsec θ. sin φ, z = c tan θ

∴		x²	+	y²	-	z²
		a²		b²		c²

= sec² θ. cos² φ + sec² θ.sin² φ –
tan² φ
= sec² θ (cos² φ + sin² φ) – tan² θ
= sec² θ – tan² θ = 1