Difference between revisions of "2021 AMC 10A Problems/Problem 4"
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==Solution 2 (Answer Choices and Modular Arithmetic)== | ==Solution 2 (Answer Choices and Modular Arithmetic)== | ||
− | From the <math>30</math>-term sum <cmath>5+12+19+26+\cdots</cmath> in Solution 1, taking modulo <math>10</math> gives <cmath>5+12+19+26+\cdots \equiv 3(0+1+2+3+4+5+6+7+8+9) = 3(45)\equiv5 \pmod{10}.</cmath> The only answer choices | + | From the <math>30</math>-term sum <cmath>5+12+19+26+\cdots</cmath> in Solution 1, taking modulo <math>10</math> gives <cmath>5+12+19+26+\cdots \equiv 3(0+1+2+3+4+5+6+7+8+9) = 3(45)\equiv5 \pmod{10}.</cmath> The only answer choices congruent to <math>5</math> modulo <math>10</math> are <math>\textbf{(A)}</math> and <math>\textbf{(D)}.</math> By a quick estimation, <math>\textbf{(A)}</math> is too small, leaving us with <math>\boxed{\textbf{(D)} ~3195}.</math> |
~MRENTHUSIASM | ~MRENTHUSIASM |
Revision as of 14:18, 25 April 2021
Contents
- 1 Problem
- 2 Solution 1 (Arithmetic Series)
- 3 Solution 2 (Answer Choices and Modular Arithmetic)
- 4 Solution 3
- 5 Video Solution (Simple and Quick)
- 6 Video Solution (Arithmetic Sequence but in a different way)
- 7 Video Solution (Using Arithmetic Sequence)
- 8 Video Solution 4
- 9 Video Solution by TheBeautyofMath
- 10 See Also
Problem
A cart rolls down a hill, travelling inches the first second and accelerating so that during each successive
-second time interval, it travels
inches more than during the previous
-second interval. The cart takes
seconds to reach the bottom of the hill. How far, in inches, does it travel?
Solution 1 (Arithmetic Series)
Since we seek the sum
in which there are
addends. The last addend is
Therefore, the requested sum is
Recall that to find the sum of an arithmetic series, we take the average of the first and last terms, then multiply by the number of terms, namely
~MRENTHUSIASM
Solution 2 (Answer Choices and Modular Arithmetic)
From the -term sum
in Solution 1, taking modulo
gives
The only answer choices congruent to
modulo
are
and
By a quick estimation,
is too small, leaving us with
~MRENTHUSIASM
Solution 3
The distance (in inches) traveled within each 1-second interval is:
This is an arithmetic sequence so the total distance travelled, found by summing them up is:
Or,
~BakedPotato66
Video Solution (Simple and Quick)
~ Education, the Study of Everything
Video Solution (Arithmetic Sequence but in a different way)
https://www.youtube.com/watch?v=sJa7uB-UoLc&list=PLexHyfQ8DMuKqltG3cHT7Di4jhVl6L4YJ&index=4
~ North America Math Contest Go Go Go
Video Solution (Using Arithmetic Sequence)
~ pi_is_3.14
Video Solution 4
~savannahsolver
Video Solution by TheBeautyofMath
https://youtu.be/50CThrk3RcM?t=262
~IceMatrix
See Also
2021 AMC 10A (Problems • Answer Key • Resources) | ||
Preceded by Problem 3 |
Followed by Problem 5 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
All AMC 10 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.