Who likes to cut it that way?

Calculus Level 2

As shown in the diagram, using 2 straight cuts from the same starting point on the perimeter, I've cut a circular pizza into 3 pieces of equal areas. However, each of the 2 identical pieces at the top and bottom has more crust than the middle piece!

A slice with larger crust has X X % more crust than the slice with smaller crust.

Assuming the crust of the pizza is 1-dimensional, find X X to the nearest integer.


The answer is 143.

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4 solutions

Zain Majumder
Feb 25, 2018

Assume the pizza has radius 1 1 .

We can find that 4 B A O = B O C 4 * \angle BAO = \angle BOC from inscribed angles. The area of the isosceles triangles A B O \triangle ABO and A C O \triangle ACO are each sin x cos x \sin{x}\cos{x} which can be found by dividing them into two smaller right triangles with hypotenuse 1 1 (see below). Using the sector area formula, the area of sector B C O BCO is 1 2 4 x 1 2 = 2 x \frac{1}{2}4x*1^2=2x . Therefore, 2 sin x cos x + 2 x = π 3 2\sin{x}\cos{x}+2x=\frac{\pi}{3} . Solving this gets x 0.268133 x \approx 0.268133 , so B O C = 4 x 1.072534 \angle BOC = 4x \approx 1.072534 . We find that B O A = C O A 2.605326 143 % \angle BOA = \angle COA \approx 2.605326 \approx \boxed{143\%} larger than B O C \angle BOC , and the same perecentage applies to the crust of each slice.

Here, D O = sin x DO = \sin{x} and A B = 2 cos x AB = 2\cos{x} , so the area is 1 2 sin x 2 cos x = sin x cos x \frac{1}{2}\sin{x}*2\cos{x}=\sin{x}\cos{x} .

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import sys
from math import cos, sin, pi

def bisection(f, x_L, x_R, eps, return_x_list=False):
    f_L = f(x_L)
    if f_L*f(x_R) > 0:
        print "Error! Function does not have opposite \
                 signs at interval endpoints!"
        sys.exit(1)
    x_M = float(x_L + x_R)/2.0
    f_M = f(x_M)
    iteration_counter = 1
    if return_x_list:
        x_list = []

    while abs(f_M) > eps:
        if f_L*f_M > 0:   # i.e. same sign
            x_L = x_M
            f_L = f_M
        else:
            x_R = x_M
        x_M = float(x_L + x_R)/2
        f_M = f(x_M)
        iteration_counter += 1
        if return_x_list:
            x_list.append(x_M)
    if return_x_list:
        return x_list, iteration_counter
    else:
        return x_M, iteration_counter

def f(x):
    return 2*cos(x)*sin(x) + 2*x - pi/3

#def f(x): # Alternatively
#    return sin(2*x) + 2*x - pi/3

a = 0;   b = 1000

solution, no_iterations = bisection(f, a, b, eps=1.0e-12)

print "Number of function calls: %d" % (1 + 2*no_iterations)
print "x = %0.12f" % (solution)

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Number of function calls: 101
x = 0.268133489494

Michael Fitzgerald - 3 years, 3 months ago

Solving x numerically using Python

Michael Fitzgerald - 3 years, 3 months ago

Hi, how did you know that the area of ABO and ACO are each sinx*cosx?

FrEshy Pisuttisarun - 3 years, 3 months ago

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I've added a picture for clarification. Since the radius is 1 1 , the legs of the right triangle are sin x \sin{x} and cos x \cos{x} .

Zain Majumder - 3 years, 3 months ago

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Awesome, thanks!

FrEshy Pisuttisarun - 3 years, 3 months ago

Or, once you found x, you could have used the formula for the arc length of a sector to derive arcBC, and then the other two arcs as well and then figured out the answer from there. But very good solution.

David Ortiz - 3 years, 3 months ago

I thought the point of the question was calculating x.

Led Tasso - 3 years, 3 months ago

So, this was to be a numerical guesswork solution... Was there any specific way to solve a hybrid equation involving trigonometric and algebraic functions??

Ananya Aaniya - 3 years, 3 months ago
David Vreken
Feb 17, 2018

We can break up the middle piece as follows, where A C AC and A D AD are the two cuts, O O is the center of the pizza circle, A B AB is the diameter, x x is the measurement of B O C \angle BOC , and r r is the radius of the pizza circle.

Then the area of sector B C BC is A s e c t o r = x 2 π π r 2 = x 2 r 2 A_{sector} = \frac{x}{2\pi}\pi r^2 = \frac{x}{2}r^2 , and the area of A O B \triangle AOB is A = 1 2 r 2 sin ( π x ) = 1 2 r 2 sin ( x ) A_{\triangle} = \frac{1}{2}r^2\sin(\pi - x) = \frac{1}{2}r^2\sin(x) . By symmetry, the area of sector B D BD is the same as the area of sector B C BC and the area of A O D \triangle AOD is the same as the area of A O B \triangle AOB .

Since all of these parts must add to one third the area of the circle, we have 1 3 π r 2 = 2 ( x 2 r 2 + 1 2 r 2 sin ( x ) ) \frac{1}{3}\pi r^2 = 2(\frac{x}{2}r^2 + \frac{1}{2}r^2\sin(x)) , which simplifies to π = 3 sin ( x ) + 3 x \pi = 3 \sin(x) + 3x . Solving numerically gives x 0.53626698 x \approx 0.53626698 .

Since the crust of the upper slice is π x \pi - x , and the crust of the middle slice is 2 x 2x , its percent increase is π x 2 x 2 x 100 % 143 % \frac{\pi - x - 2x}{2x} \cdot 100\% \approx \boxed{143}\% .

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import sys
from math import cos, sin, pi

def bisection(f, x_L, x_R, eps, return_x_list=False):
    f_L = f(x_L)
    if f_L*f(x_R) > 0:
        print "Error! Function does not have opposite \
                 signs at interval endpoints!"
        sys.exit(1)
    x_M = float(x_L + x_R)/2.0
    f_M = f(x_M)
    iteration_counter = 1
    if return_x_list:
        x_list = []

    while abs(f_M) > eps:
        if f_L*f_M > 0:   # i.e. same sign
            x_L = x_M
            f_L = f_M
        else:
            x_R = x_M
        x_M = float(x_L + x_R)/2
        f_M = f(x_M)
        iteration_counter += 1
        if return_x_list:
            x_list.append(x_M)
    if return_x_list:
        return x_list, iteration_counter
    else:
        return x_M, iteration_counter

def f(x):
    return 3*sin(x) + 3*x - pi

a = 0;   b = 1000

solution, no_iterations = bisection(f, a, b, eps=1.0e-12)

print "Number of function calls: %d" % (1 + 2*no_iterations)
print "x = %0.12f" % (solution)

1
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Number of function calls: 103
x = 0.536266978989

Michael Fitzgerald - 3 years, 3 months ago

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My solution solves pi = 3 sin x + 3x, not pi = 3 cos x sin x + 3x

David Vreken - 3 years, 3 months ago

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My bad! I will fix original instead of reposting here. Tx for pointing that out

Michael Fitzgerald - 3 years, 3 months ago

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@Michael Fitzgerald No worries!

David Vreken - 3 years, 3 months ago

My solution x ~= 0.586311970336 is incorrect and correctly pointed out by David x ~= 0.536266978989 as shown in solution above.

Michael Fitzgerald - 3 years, 3 months ago
Meneghin Mauro
Feb 28, 2018

Calling 2 θ 2\theta the angle of the smallest arch seen by the centre of the pizza, expressing the area of the central slice as function of θ \theta reduces to solving numerically θ + sin θ = π 3 \theta+\sin\theta=\frac{\pi}{3}

I've done this iteratively

θ 0 = π 3 \theta_0=\frac{\pi}{3}

θ i + 1 = θ i + π / 3 sin θ i 2 \theta_{i+1}=\frac{\theta_i + \pi/3 - \sin\theta_i}{2}

After 9 iterations this gives 0.536266979

The rest is straightforward, X = π 3 θ 2 θ 100 = 143 X=\frac{\pi-3\theta}{2\theta}*100=143

M N
Feb 28, 2018

The area of the top piece is the area of a segment of a circle. This can be found by taking the area of the sector that contains it and subtracting the area of the triangle in that same sector. Let's call the central angle of this sector measured in radians x. Area of sector

1/2 x * r^2

Area of triangle

1/2 * 2 sin(x/2) * cos(x/2) * r^ 2=

1/2 sin(x) r^2

Let's say r = 1 to simplify calculations The area of this segment has to be equal to 1/3 of the total area, which is pi/3

Area of sector - Area of triangle

x - sin(x) = 2pi/3

x approximately = 2.605 The crust of this segment is x And the crust of the large segment is 2pi - 2x Solving for both of these, finding the percent increase, and rounding to the nearest integer gives

143

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