# Math 21a: section knill Fall 2022

## 21a Fall 2022

# Open AI II, See part I and See part III

We experimented a bit more with Open AI.## 1. Basic algebra. Especially PEMDAS

This is a famous problem ( see this page)What is 2x/3y-1 if x=9 and y=2? Open AI Answer: 2x/3y-1 = (2*9)/(3*2)-1 = 6/6-1 = 0 Grading: This is a big surprise because the AI only got an inexplicable basic arithmetic error and not any other issues. It was almost there and should have got 2x/3y-1 = (2*9)/(3*2)-1 = 18/6-1 = 2 which is the answer most humans get. If we ask Compute (2*9)/(3*2)-1 the AI gets indeed 2. This problem came up, because almost all computers and programming languages give 11. Here in mathematica x = 9; y = 2; 2x/3y - 1 which gives the answer 11.

## 2. Basic calculus

This is still disappointing. We would expect that the AI would just feed it into a computer algebra system:What is the anti derivative of exp(-x^2) Open AI Answer: The anti derivative of exp(-x^2) is -1/2 exp(-x^2) + C, where C is an arbitrary constant. This is completely off.

## 3. Multivariable calculus exam problems

Practice Exam 1 Problem 2, Math 21a, Fall 2022 Find the global maximum and minimum values of the function f(x,y,z) = x z + y^2+ 2 y on the region x^2+y^2+z^2 <=4. Open AI Answer: Global max 8, global min 0, Global Maximum: 8 TRUE Global Minimum: 0 FALSE -8/3 is the minimumhere is a generic word problem. The formulation helps a bit

Practice Exam 1 Problem 6, Math 21a, Fall 2022 The post office will only ship cuboid boxes of dimension x,y,z for which the height plus girth z+2x+2y does not exceed 108. What dimensions will result in the greatest volume x*y*z? Open AI Answer: The greatest volume will be achieved when x, y, and z are all equal. In this case, the maximum dimensions would be x = y = z = 36, resulting in a volume of 46,656. Grading: Answer is wrong. This is a Lagrange problem. The best is (x,y,z) = (18,18,38) f=x*y*z; g=z+2x+2y; c=108; Solve[{D[f,x]==L*D[g,x], D[f,y]==L*D[g,y], D[f,z]==L*D[g,z], g==c},{x,y,z,L}]Here is a True false problem

True False Practice Final Exam 3 problem, problem 8a For every scalar function f there is a vector field F such that div(F)=f. Open AI Answer: Yes, this is true. The vector field F is called the gradient of f and is defined as F = ∇f. The divergence of the gradient of a scalar function is equal to the scalar function itself, so div(F) = div(∇f) = f. Grading: Correct answer but completely wrong explanation. div grad (f) = 0 can not be f. The trick is to restrict F to F=[P,0,0] and solve P_x = f.And an other one

True False Practice Final Exam 3 problem, problem 8b If f(x,y) is a function with no critical points, then f cannot attain a global maximum on x^2+y^2 <= 1. Open AI Answer: This is not necessarily true. A function with no critical points can still have a global maximum on x^2+y^2 <= 1. For example, the function f(x,y) = x^2 + y^2 has no critical points, but it has a global maximum of 1 on x^2+y^2 <= 1. Grading: This is correct. A better explanation would be to cite the extremal value theorem which assures that a continuous function on a closed and bounded domain attains a maximum and minimum.

## Putnam 2022 exam problems

We only looked at some problems where we suspect success is possible:Putnam Exam problem A1 2022 Determine all ordered pairs of real numbers (a, b) such that the line y = ax+b intersects the curve y = ln(1+ x^2 ) in exactly one point. Open AI Answer: (1, -1) and (-1, 1). Answer is partially correct. There are more examples like (0,0) Plot[ {1 - x, x - 1, Log[1 + x^2]}, {x, -5, 5}]An other putnam

Putnam Exam Problem B6 2022 Find all continuous functions f from the positive real numbers to the positive real numbers such that f(x f(y))+ f(y f(x)) = 1+ f(x+y) for all positive x and y. Open AI Answer The only continuous function that satisfies this equation is f(x) = 1. Answer is partially correct. There are more examplesHere is a tougher problem:

Putnam Exam Problem A5 2022 Alice and Bob play a game on a board consisting of one row of 2022 consecutive squares. They take turns placing tiles that cover two adjacent squares, with Alice going first. By rule, a tile must not cover a square that is already covered by another tile. The game ends when no tile can be placed according to this rule. Alice's goal is to maximize the number of uncovered squares when the game ends; Bob's goal is to minimize it. What is the greatest number of uncovered squares that Alice can ensure at the end of the game, no matter how Bob plays? Open AI Answer: The greatest number of uncovered squares that Alice can ensure at the end of the game is 1011. Grading: Answer is false. Bob can put many tiles in which are not adjacent. The correct answer is 290. There is a recursion for the optimal final score a(n) of Alice and the optimal final score b(n) of Bob. We have a(n) = f(n)+a(n-f(n)), b(n) = f(n)+b(n-f(n)), with f(n)=floor(n/7)

## Problem solving examples

Problem in Polya's How to solve on page 38: Find the center of gravity of a homogenous tetrahedron. Open AI answer: The center of gravity of a homogenous tetrahedron is located at the intersection of the three medians, which is also the centroid of the tetrahedron. The centroid is the point where the four vertices of the tetrahedron meet and is located at the center of the tetrahedron. Grading: correct answer.Here are some tougher cookies:

Problem 2.5 in Tao's book Solving mathematical problems Prove that for any non-negative integer n the number 1^n+2^n+3^n+4^n is divisible by 5 if and only if n is not divisible by 4 Open AI Answer: First, assume that n is not divisible by 4. We can write 1^n+2^n+3^n+4^n as (1+2+3+4)^n. Since n is not divisible by 4, (1+2+3+4)^n is divisible by 5. Grading: Proof is false. It is not true that 1^n+2^n+3^n+4^n=(1+2+3+4)^n if n is not divisble by 4. For example n = 3, 1^n + 2^n + 3^n + 4^n=100.And an other one from that book

Problem 2.2 in Tao's book Solving mathematical problems Show that there is no power of 2 such that its decimal digits could be rearranged and made into another power of 2. No zeroes are allowed in leading digits. Open AI Answer: We will prove this by contradiction. Suppose there is a power of 2 such that its decimal digits can be rearranged and made into another power of 2. Let the power of 2 be 2^n and the rearranged power of 2 be 2^m. Since the rearranged power of 2 is also a power of 2, we know that m = log_2(2^m) = log_2(2^n) = n. This means that the Grading: Proof is not only incomplete but also can not be done. We need an invariant like digit sum hash. This is periodic with period 6 with no repetition. But since 2^6 is larger than 10 the number of digits can not be the same.