Lesson Worksheet: Directional Derivatives and Gradient Mathematics

In this worksheet, we will practice finding a derivative of multivariable functions in a given direction (directional derivative) and finding the gradient vector of the function.

Q1:

Find a function 𝑄(𝑥,𝑦) so that the vector field F(𝑥,𝑦)=𝑥𝑦,𝑄(𝑥,𝑦) is a gradient field.

  • A𝑄(𝑥,𝑦)=𝑥𝑦
  • B𝑄(𝑥,𝑦)=𝑥
  • C𝑄(𝑥,𝑦)=𝑥𝑦2
  • D𝑄(𝑥,𝑦)=𝑥2
  • E𝑄(𝑥,𝑦)=𝑦2

Q2:

Suppose 𝑤=𝐹(𝜙(𝑥,𝑦)) with 𝜙=(𝑥+𝑦,𝑥𝑦,𝑥𝑦). Express the gradient 𝑤𝜋,23 (viewed as a 1×2 matrix) in terms of the 1×3 matrix 𝐹(𝑞), where 𝑞=𝜙𝜋,23, and a matrix of partial derivatives of 𝜙.

  • A𝑤𝜋,23=𝐹(𝑞)2𝜋432𝜋4323𝜋
  • B𝑤𝜋,23=𝐹(𝑞)2𝜋432𝜋4323𝜋
  • C𝑤𝜋,23=𝐹(𝑞)2𝜋432𝜋4323𝜋
  • D𝑤𝜋,23=𝐹(𝑞)𝜋23𝜋2323𝜋
  • E𝑤𝜋,23=𝐹(𝑞)𝜋23𝜋2323𝜋

Q3:

Suppose 𝑤=𝐹(𝜙(𝑥,𝑦)) with 𝜙=(𝜙,𝜙,𝜙) and that 𝑞=𝜙(𝑝) for a point 𝑝. Express the gradient 𝑤(𝑝) (viewed as a 1×2 matrix) in terms of the 1×3 matrix 𝐹(𝑞) and a matrix of partial derivatives of 𝜙.

  • A𝑤(𝑝)=𝐹(𝑞)𝜕𝜙𝜕𝑦𝜕𝜙𝜕𝑦𝜕𝜙𝜕𝑦𝜕𝜙𝜕𝑥𝜕𝜙𝜕𝑥𝜕𝜙𝜕𝑥
  • B𝑤(𝑝)=𝐹(𝑞)𝜕𝜙𝜕𝑥𝜕𝜙𝜕𝑥𝜕𝜙𝜕𝑥𝜕𝜙𝜕𝑦𝜕𝜙𝜕𝑦𝜕𝜙𝜕𝑦
  • C𝑤(𝑝)=𝐹(𝑞)𝜕𝜙𝜕𝑥𝜕𝜙𝜕𝑦𝜕𝜙𝜕𝑥𝜕𝜙𝜕𝑦𝜕𝜙𝜕𝑥𝜕𝜙𝜕𝑦
  • D𝑤(𝑝)=𝐹(𝑞)𝜕𝜙𝜕𝑦𝜕𝜙𝜕𝑥𝜕𝜙𝜕𝑦𝜕𝜙𝜕𝑥𝜕𝜙𝜕𝑦𝜕𝜙𝜕𝑥
  • E𝑤(𝑝)=𝐹(𝑞)𝜕𝜙𝜕𝑥𝜕𝜙𝜕𝑦𝜕𝜙𝜕𝑥𝜕𝜙𝜕𝑦𝜕𝜙𝜕𝑥𝜕𝜙𝜕𝑦

Q4:

Let 𝑤=𝐹(𝜙(𝑥,𝑦)) where 𝜙=𝑥+𝑦,𝑥𝑦,𝑥𝑦 and 𝐹(𝑝,𝑞,𝑟)=10𝑝+6𝑞16𝑟. Given that there is a line in the 𝑥-𝑦 plane for which 𝑤=0, find the equation of this line.

  • A𝑦=𝑥
  • B𝑦=2𝑥
  • C𝑦=𝑥2
  • D𝑦=2𝑥
  • E𝑦=𝑥

Q5:

The chain rule for the composition of a curve 𝜙(𝑡)=(𝑥(𝑡),𝑦(𝑡)) and a function 𝑓(𝑥,𝑦) says that dd𝑡𝑓(𝜙(𝑡))=𝑓𝜙(𝑡), where 𝑓(𝑥,𝑦) is the gradient of 𝑓. We have been given the function 𝑓(𝑥,𝑦)=𝑥+𝑦 and the curve 𝜙(𝑡)=((2𝑡),(𝑡))sincos. At certain points, such as the origin, or points 𝐴 and 𝐵 in the figure, dd𝑡(𝑓(𝜙(𝑡)))=0.

Find the parameters 𝑡,𝑡(0,2𝜋] for 𝐴 and 𝐵. Leave your answer in terms of cos.

  • A𝑡=1214cos, 𝑡=2𝜋
  • B𝑡=12cos, 𝑡=𝜋2
  • C𝑡=1212cos, 𝑡=2𝜋
  • D𝑡=1412cos, 𝑡=𝜋
  • E𝑡=14cos, 𝑡=𝜋2

Q6:

Suppose 𝑤=𝐹(𝜙(𝑠,𝑡)) with 𝜙=(𝜙,𝜙,𝜙) given by the linear mapping 𝜙(𝑠,𝑡)=(𝑎𝑠+𝑏𝑡,𝑐𝑠+𝑑𝑡,𝑒𝑠+𝑓𝑡), where 𝑎,,𝑓 are constants.

What is 𝜙(𝑠,𝑡)?

  • A𝑒𝑠+𝑓𝑡
  • B𝑐𝑠+𝑑𝑡
  • C𝑎𝑠+𝑏𝑡

If we consider the domains of 𝜙 and of 𝐹 as consisting of column vectors, then 𝜙 can be written as a multiplication by a matrix. What is this matrix?

  • A𝑎𝑑𝑏𝑒𝑐𝑓
  • B𝑎𝑐𝑒𝑏𝑑𝑓
  • C𝑏𝑑𝑓𝑎𝑐𝑒
  • D𝑏𝑎𝑑𝑐𝑓𝑒
  • E𝑎𝑏𝑐𝑑𝑒𝑓

There are 6 partial derivatives of 𝜙. Evaluate 𝜕𝜙𝜕𝑡𝜋,23.

  • A𝑒
  • B𝜋𝑠23𝑓
  • C𝑓
  • D𝜋𝑠+𝑓
  • E23𝑓

The gradient 𝐹 of 𝐹 at the point (1,2,3) of can also be thought of as a linear map into . What is the corresponding 1×3 matrix?

  • A𝜕𝐹𝜕𝑧(1,2,3)𝜕𝐹𝜕𝑦(1,2,3)𝜕𝐹𝜕𝑥(1,2,3)
  • B𝜕𝐹𝜕𝑦(1,2,3)𝜕𝐹𝜕𝑧(1,2,3)𝜕𝐹𝜕𝑥(1,2,3)
  • C𝜕𝐹𝜕𝑥(1,2,3)𝜕𝐹𝜕𝑦(1,2,3)𝜕𝐹𝜕𝑧(1,2,3)
  • D𝜕𝐹𝜕𝑦(1,2,3)𝜕𝐹𝜕𝑧(1,2,3)𝜕𝐹𝜕𝑥(1,2,3)
  • E𝜕𝐹𝜕𝑥(1,2,3)𝜕𝐹𝜕𝑦(1,2,3)𝜕𝐹𝜕𝑧(1,2,3)

Now 𝑤=𝐹(𝑎𝑠+𝑏𝑡,𝑐𝑠+𝑑𝑡,𝑒𝑠+𝑓𝑡) is a function from to with a gradient represented by a 1×2 matrix. Writing 𝑝 for 𝜙𝜋,23, what is this matrix at the point 𝜋,23?

  • A𝑎𝜕𝐹𝜕𝑥(𝑝)+𝑏𝜕𝐹𝜕𝑦(𝑝)+𝑐𝜕𝐹𝜕𝑧(𝑝)𝑑𝜕𝐹𝜕𝑥(𝑝)+𝑒𝜕𝐹𝜕𝑦+𝑓𝜕𝐹𝜕𝑧(𝑝)
  • B𝑏𝜕𝐹𝜕𝑥(𝑝)+𝑑𝜕𝐹𝜕𝑦(𝑝)+𝑓𝜕𝐹𝜕𝑧(𝑝)𝑎𝜕𝐹𝜕𝑥(𝑝)+𝑐𝜕𝐹𝜕𝑦+𝑒𝜕𝐹𝜕𝑧(𝑝)
  • C𝑎𝜕𝐹𝜕𝑥(𝑝)+𝑐𝜕𝐹𝜕𝑦(𝑝)+𝑒𝜕𝐹𝜕𝑧(𝑝)𝑏𝜕𝐹𝜕𝑥(𝑝)+𝑑𝜕𝐹𝜕𝑦+𝑓𝜕𝐹𝜕𝑧(𝑝)
  • D𝑎𝜕𝐹𝜕𝑥(𝑝)+𝑐𝜕𝐹𝜕𝑦(𝑝)+𝑒𝜕𝐹𝜕𝑧(𝑝)𝑏𝜕𝐹𝜕𝑥(𝑝)+𝑑𝜕𝐹𝜕𝑦+𝑓𝜕𝐹𝜕𝑧(𝑝)
  • E𝑏𝜕𝐹𝜕𝑥(𝑝)+𝑑𝜕𝐹𝜕𝑦(𝑝)+𝑓𝜕𝐹𝜕𝑧(𝑝)𝑎𝜕𝐹𝜕𝑥(𝑝)+𝑐𝜕𝐹𝜕𝑦+𝑒𝜕𝐹𝜕𝑧(𝑝)

Write a matrix equation relating 𝑤𝜋,23 with 𝐹(𝑝).

  • A𝑤𝜋,23=𝐹(𝑝)𝑎𝑑𝑏𝑒𝑐𝑓
  • B𝑤𝜋,23=𝐹(𝑝)𝑎𝑏𝑐𝑑𝑒𝑓
  • C𝑤𝜋,23=𝐹(𝑝)𝑏𝑎𝑑𝑐𝑓𝑒
  • D𝑤𝜋,23=𝐹(𝑝)𝑎𝑐𝑒𝑏𝑑𝑓
  • E𝑤𝜋,23=𝐹(𝑝)𝑏𝑑𝑓𝑎𝑐𝑒

Practice Means Progress

Download the Nagwa Practice app to access questions, unit-exams, and flashcards for your school courses.

scan me!

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.