Difference between revisions of "2011 AMC 10A Problems/Problem 4"
(→Problem) |
m (→Solution 4) |
||
(4 intermediate revisions by 3 users not shown) | |||
Line 30: | Line 30: | ||
We see that every number in Y's sequence is two more than every corresponding number in X's sequence. Since there are 46 numbers in each sequence, the difference must be: | We see that every number in Y's sequence is two more than every corresponding number in X's sequence. Since there are 46 numbers in each sequence, the difference must be: | ||
<math>46\cdot 2=\boxed{92}</math> | <math>46\cdot 2=\boxed{92}</math> | ||
+ | |||
+ | ==Solution 3== | ||
+ | |||
+ | <cmath>\begin{align*} | ||
+ | X&=10+12+14+\cdots +100 \\ | ||
+ | Y&=X-10+102 = X+92 \\ | ||
+ | Y-X &= (X+92)-X \\ | ||
+ | Y-X &= X-X+92 \\ | ||
+ | Y-X &= 0+92 \\ | ||
+ | Y-X &= \boxed{92} \quad \quad \textbf{(A)}\\ | ||
+ | \end{align*} </cmath> | ||
+ | <math>\blacksquare</math> | ||
+ | |||
+ | - <math>\text{herobrine-india}</math> | ||
+ | |||
+ | ==Solution 4== | ||
+ | |||
+ | In an actual contest, this would probably take too much time but is nevertheless a solution. | ||
+ | The general formula for computing sums of any arithmetic sequence where x is the number of terms, f is the first term and l is the last term is ((f+l)/2)*x. If one uses that formula for both sequences, they will get 2530 as the sum for X and 2622 as the sum for Y. | ||
+ | Subtracting Y-X, one will get the answer, 92. | ||
+ | - danfan | ||
+ | |||
+ | ==Video Solution== | ||
+ | https://youtu.be/L6utIF9FzPQ | ||
+ | |||
+ | ~savannahsolver | ||
== See Also == | == See Also == | ||
{{AMC10 box|year=2011|ab=A|num-b=3|num-a=5}} | {{AMC10 box|year=2011|ab=A|num-b=3|num-a=5}} | ||
{{MAA Notice}} | {{MAA Notice}} |
Revision as of 12:36, 11 September 2021
Contents
Problem
Let X and Y be the following sums of arithmetic sequences:
What is the value of
Solution 1
We see that both sequences have equal numbers of terms, so reformat the sequence to look like:
From here it is obvious that .
Note
Another way to see this is to let the sum So, the sequences become
Like before, the difference between the two sequences is
Solution 2
We see that every number in Y's sequence is two more than every corresponding number in X's sequence. Since there are 46 numbers in each sequence, the difference must be:
Solution 3
-
Solution 4
In an actual contest, this would probably take too much time but is nevertheless a solution. The general formula for computing sums of any arithmetic sequence where x is the number of terms, f is the first term and l is the last term is ((f+l)/2)*x. If one uses that formula for both sequences, they will get 2530 as the sum for X and 2622 as the sum for Y. Subtracting Y-X, one will get the answer, 92. - danfan
Video Solution
~savannahsolver
See Also
2011 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.