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91Ó°ÊÓ

In Exercises 21–26, find the discriminant of the given function.

fθ,ϕ=cosθsinϕ.

Short Answer

Expert verified

The answer issin2ϕcos2θ-cos2ϕsin2θ.

Step by step solution

01

Step 1. Given Information.

The function isfθ,ϕ=cosθsinϕ.

02

Step 2. Explanation.

The discriminant is calculated by formula,

detHf=∂2f∂θ2·∂2f∂ϕ2-∂2f∂θ∂ϕ2.

Calculate ∂f∂θ, ∂f∂∅, ∂2f∂θ2, ∂2f∂ϕ2and ∂2f∂θ∂ϕ.

∂f∂θ=-sinθsinϕ, ∂2f∂θ2=-cosθsinϕ.

localid="1650041459981" ∂f∂∅=cosϕcosθ, ∂2f∂ϕ2=-sinϕcosθ.

∂2f∂θ∂ϕ=-cosϕsinθ.

03

Step 3. Calculation.

Calculate detHf=∂2f∂θ2·∂2f∂ϕ2-∂2f∂θ∂ϕ2.


role="math" localid="1650042389735" detHf=-cosθsinϕ-sinϕcosθ--cosϕsinθ2=sin2ϕcos2θ-cos2ϕsin2θ

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