Difference between revisions of "2020 AMC 10B Problems/Problem 9"

Revision as of 16:31, 6 April 2021

The following problem is from both the 2020 AMC 10B #9 and 2020 AMC 12B #8, so both problems redirect to this page.

Problem

How many ordered pairs of integers $(x, y)$ satisfy the equation $x^{2020}+y^2=2y?$

$\textbf{(A) } 1 \qquad\textbf{(B) } 2 \qquad\textbf{(C) } 3 \qquad\textbf{(D) } 4 \qquad\textbf{(E) } \text{infinitely many}$

Solutions

Solution 1

Rearranging the terms and and completing the square for $y$ yields the result $x^{2020}+(y-1)^2=1$ . Then, notice that $x$ can only be $0$ , $1$ and $-1$ because any value of $x^{2020}$ that is greater than 1 will cause the term $(y-1)^2$ to be less than $0$ , which is impossible as $y$ must be real. Therefore, plugging in the above values for $x$ gives the ordered pairs $(0,0)$ , $(1,1)$ , $(-1,1)$ , and $(0,2)$ gives a total of $\boxed{\textbf{(D) }4}$ ordered pairs.

Solution 2

Bringing all of the terms to the LHS, we see a quadratic equation $y^2 - 2y + x^{2020} = 0$ in terms of $y$ . Applying the quadratic formula, we get $y = \frac{2\pm\sqrt{4-4\cdot1\cdot x^{2020}}}{2}=\frac{2\pm\sqrt{4(1-x^{2020})}}{2}.$ In order for $y$ to be real, which it must be given the stipulation that we are seeking integral answers, we know that the discriminant, $4(1-x^{2020})$ must be nonnegative. Therefore, $4(1-x^{2020}) \geq 0 \implies x^{2020} \leq 1.$ Here, we see that we must split the inequality into a compound, resulting in $-1 \leq x \leq 1$ .

The only integers that satisfy this are $x \in \{-1,0,1\}$ . Plugging these values back into the quadratic equation, we see that $x = \{-1,1\}$ both produce a discriminant of $0$ , meaning that there is only 1 solution for $y$ . If $x = \{0\}$ , then the discriminant is nonzero, therefore resulting in two solutions for $y$ .

Thus, the answer is $2 \cdot 1 + 1 \cdot 2 = \boxed{\textbf{(D) }4}$ .

~Tiblis

Solution 3: Solve for x first

Set it up as a quadratic in terms of y: $y^2-2y+x^{2020}=0$ Then the discriminant is $\Delta = 4-4x^{2020}$ This will clearly only yield real solutions when $|x^{2020}| \leq 1$ , because the discriminant must be positive. Then $x=-1,0,1$ . Checking each one: $-1$ and $1$ are the same when raised to the 2020th power: $y^2-2y+1=(y-1)^2=0$ This has only has solutions $1$ , so $(\pm 1,1)$ are solutions. Next, if $x=0$ : $y^2-2y=0 \Rightarrow y(y-2)=0$ Which has 2 solutions, so $(0,2)$ and $(0,0)$ .

These are the only 4 solutions, so our answer is $\boxed{\textbf{(D) } 4}$ .

~edits by BakedPotato66

Solution 4: Solve for y first

Move the $y^2$ term to the other side to get $x^{2020}=2y-y^2 = y(2-y)$ .

Because $x^{2020} \geq 0$ for all $x$ , then $y(2-y) \geq 0 \Rightarrow y = 0,1,2$ .

If $y=0$ or $y=2$ , the right side is $0$ and therefore $x=0$ .

When $y=1$ , the right side become $1$ , therefore $x=1,-1$ .

Our solutions are $(0,2)$ , $(0,0)$ , $(1,1)$ , $(-1,1)$ . These are the only $4$ solutions, so the answer is $\boxed{\textbf{(D) } 4}$

- wwt7535

~ edits by BakedPotato66

Video Solutions

Video Solution 1

https://youtu.be/6ujfjGLzVoE

~IceMatrix

Video Solution 2

https://youtu.be/7dQ423hhgac

~savannahsolver

Video Solution 3

https://youtu.be/zfChnbMGLVQ?t=4251

~ pi_is_3.14

@@ Line 25: / Line 25: @@
 Then the discriminant is
 <cmath>\Delta = 4-4x^{2020}</cmath>
-This will clearly only yield real solutions when <math>|x^{2020}| \leq 1</math>, because it is always positive.
+This will clearly only yield real solutions when <math>|x^{2020}| \leq 1</math>, because the discriminant must be positive.
 Then <math>x=-1,0,1</math>. Checking each one:
 <math>-1</math> and <math>1</math> are the same when raised to the 2020th power:
@@ Line 31: / Line 31: @@
 This has only has solutions <math>1</math>, so <math>(\pm 1,1)</math> are solutions.
 Next, if <math>x=0</math>:
-<cmath>y^2-2y=0</cmath>
+<cmath>y^2-2y=0 \Rightarrow y(y-2)=0</cmath>
 Which has 2 solutions, so <math>(0,2)</math> and <math>(0,0)</math>.
-These are the only 4 solutions, so <math>\boxed{\textbf{(D) } 4}</math>
+These are the only 4 solutions, so our answer is <math>\boxed{\textbf{(D) } 4}</math>.
 ~edits by BakedPotato66

2020 AMC 10B (Problems • Answer Key • Resources)
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All AMC 10 Problems and Solutions

2020 AMC 12B (Problems • Answer Key • Resources)
Preceded by Problem 7	Followed by Problem 9
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All AMC 12 Problems and Solutions

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