When first introduced to multiplication in algebra, a common mistake made by students is to claim that a × ( b + c ) = a × b + c , because they forget to distribute a properly. For how many ordered triples of integers ( a , b , c ) , each of which are between 0 and 10 inclusive, do we actually have a × ( b + c ) = a × b + c ?
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
good procedure
I prescribe you approach to everyone
Best approach
Gud 1.. I did it in the same way :)
Wow! I was so close! I forgot to subtract 11(my answer was 242) Thanks for the answer!
Log in to reply
Yeah, I actually put 241 at first because I subtracted 1 instead of 11.
You write "since it ranges from 0 to 10 inclusive". However the problem states "between 0 and 10." Clearly, the problem agrees with your interpretation but that's what tripped me up.
Log in to reply
Yeah, but in the problem, it states that each variable is from 0-10 inclusive.
same way !!!
is ur answer correct?
By expanding the left-hand side, we get a b + a c = a b + c Subtracting ab from each side gives us a c = c Subtracting c from each side gives us a c − c = 0 Taking out a factor of c gives us c ( a − 1 ) = 0 Now we know that either c = 0 or a − 1 = 0 For each of these there are 11 options (each of the integers between 0 and 10 inclusive) for the other variable giving a total of 22. However, we must be careful to remove 1 instance of a = 1 , c = 0 as this is duplicated. This gives us 21 different combinations for a and c. For each of the combinations, there are again 11 different possible values for b. Therefore the final answer is 2 1 × 1 1 = 2 3 1
Thanks. I felt it was clearest to do it this way which requires a similar thought process to solving a quadratic equation or any other factorised equation.
I did it similar to William but I think your solution actually does explain why we took those cases in the first place.
i this is a better method
Since, a × ( b + c ) = a × b + c is required Therefore, a × c = c => ( a − 1 ) × c = 0 Hence, a = 1 or c = 0
Here, b can take 11 values and c can take 11 values => 1 × 1 1 × 1 1 = 121 triplets.
In this case, a cannot be taken to be 1 because we have already considered that in previous case.
Hence , a can take 10 values and b can take 11 values => 1 0 × 1 1 × 1 = 110 triplets.
Therefore, total triplets = 231
#include <string.h>
main()
{
int a,b,c,d,e, count;
count=0;
for(a=0;a<11; a++)
{
for(b=0;b<11; b++)
{
for(c=0;c<11; c++)
{
d=a*b+a*c;
e=a*b+c;
if(d==e)
count++;
}
}
}
printf("%d", count);
}
So the solution is 2 3 1
Expanding, we get a b + a c = a b + c so a c = c Then, b can be anything, giving 11 possibilities for b .
If c = 0 , then a can be anything, giving 11 possibilities. If c = 0 , then a = 1 , and c can be anything except 0, giving 10 possibilities.
The answer is 1 1 ( 1 1 + 1 0 ) = 2 3 1 .
Can you elaborate your logic?
I mean the last step 11 (11 + 10) = 231. How you got?
Log in to reply
b can be anything, giving the first 11.
Cases for a and c are either c = 0 or a = 1 or both. The number of ways this can happen is 11+11 (11 for each) -1 (we overcount if both).
We have: a(b+c) = ab +c
Expanding, ab + ac = ab + c
ac = c
This only occurs when a = 1, or c = 0.
If a = 1, b and c both range from 0 to 10 (11 possibilities each), giving us 11 x 11 = 121 ordered triples.
If c = 0, a and b both range from 0 to 10 (11 possibilities each), giving us 11 x 11 = 121 ordered triples.
Now, all that is left is to check which triples are repeated between the two sets. Obviously, the repeated triples will be the ones where a = 1 and c = 0. Hence, b ranges from 0 to 10 (11 possibilities).
The total number of ordered triples will be 121 + 121 - 11 = 231.
The equation is true when a=1 or c=0
for a=1 -- P(a), we have 11x11 = 121 possibilities (11 from b and 11 from c)
for c=0 -- P(c), we have 11x11 = 121 possibilities (11 from a and 11 from b)
for a=1 AND c=0 -- P(a and c), we have 11 possibilities
So for a=1 OR c=0 -- P(a or c) = P(a) + P(c) - P(a and c) = 121 + 121 -11 = 231
Distributing a ( b + c ) = a b + c gives us a b + a c = a b + c ,
or a c = c ,
so a=1 or c=0
for all c=0 we have 121 ordered pairs from (0,0,0) to (0,10,0) (11 ordered pairs for each value of a, and we have 11 values from 0 to 10 inclusive)
for all a=1, c=/=0 we have another 110 ordered pairs from (1,0,1) to (1,10,1) (11 ordered pairs for each value of b, and we have 10 values from 1 to 10 inclusive)
Therefore our final answer is 1 2 1 + 1 1 0 = 2 3 1
a × ( b + c ) = a × b + a × c = a × b + c ∴ a × c = c If c = 0 ⇒ a = 1 (I) If c = 0 ⇒ a can be anything! (II)
(I) 1 × 1 1 × 1 0 = 1 1 0 (II) 1 1 × 1 1 × 1 = 1 2 1 → 1 1 0 + 1 2 1 = 2 3 1
Let's first consider these sets : X = { 0 , 1 , 2 , 3 , … , 1 0 } , Y = { 1 , 2 , 3 , … , 1 0 } Now : ∵ a × ( b + c ) = a × b + a × c according to the right distributive property in multiplication , and ∵ a × ( b + c ) = a × b + c (given) . ∴ a × b + a × c = a × b + c Then by subtracting ( a × b ) from both sides (there is no problem in that) , we get : a × c = c Now let's assume that c = 0 that would be much easier , isn't it ?
Case 1 : ( c = 0 ) : ∴ a = c c = 1 , in this case we have a = 1 , b ∈ X but c ∈ Y But what if c = 0 ?
Case 2 : ( c = 0 ) : By substituting c we get : a × 0 = 0 and this is satisfying , in this case we have a , b ∈ X , c = 0 .
Now in case 1 as we have a = 1 fixed in the ordered triple ( a , b , c ) = ( 1 , b , c ) so we look for the ordered pairs ( b , c ) , so we get the product set of X × Y which contains 1 1 × 1 0 = 1 1 0 possible ordered pairs for ( b , c ) and putting 1 at the first of each ordered pair we have 1 1 0 ordered triples .
In case 2 the same but we have c = 0 fixed in the ordered triple ( a , b , c ) = ( a , b , 0 ) so we look for the ordered pairs ( a , b ) , so we get the product set of X × X which contains 1 1 × 1 1 = 1 2 1 possible ordered pairs for ( a , b ) and putting 0 at the end of each ordered pair we have 1 2 1 ordered triples .
Summing all triples up ⇒ 1 1 0 + 1 2 1 = 2 3 1 ordered triples .
:) Yay!
Log in to reply
Pleased to see that you liked it ^_^ .
Isn't this much of an over-rated problem ? Simple casework, I guess level 3.
We can simplify a ⋅ ( b + c ) = a ⋅ b + c into a ⋅ b + a ⋅ c = a ⋅ b + c which is further simplified to a ⋅ c = c This will only be true if a = 1 , or c = 0 , or both. Let's count the cases:
Case 1 : Only a = 1 . Then, for the ordered triple a , b , c , we have 1 choice for a , (it can only be 1 ), 1 1 choices for b , (it can be any integer between 0 and 1 1 ) and 1 0 choices for c (it can be any integer between 1 and 1 0 ; just not 0 ). 1 ⋅ 1 1 ⋅ 1 0 = 1 1 0
Case 2 : Only c = 0 . Then for the ordered triple a , b , c , we have 1 0 choices for a , (it can be any integer between 0 and 1 0 ; just not 1 ), 1 1 choices for b , (it can be any integer between 0 and 1 1 ) and 1 choice for c (it can be 0 ). 1 0 ⋅ 1 1 ⋅ 1 = 1 1 0
Case 3 : Both a = 1 and c = 0 . Then for the ordered triple a , b , c , we have 1 choice for a , (it can only be 1 ), 1 1 choices for b , (it can be any integer between 0 and 1 1 ) and 1 choice for c (it can be 0 ). 1 ⋅ 1 1 ⋅ 1 = 1 1
Adding these 3 numbers together, we have 1 1 0 + 1 1 0 + 1 1 = 2 3 1 different ordered triples of ( a , b , c that satisfy a ⋅ ( b + c ) = a ⋅ b + c if ( a , b , and c are all between 0 and 1 0 inclusive.
So expanding you have
a
b
+
a
c
=
a
b
+
c
You can cancel a b out to get
a c = c
From here you have 2 paths- either a = 1 or c = 0 and a can be any value. After this it basically becomes a Combinatorics problem.
When c = 0 you can 1 1 ∗ 1 1 = 1 2 1 ways of picking a and b since we have 0 ≤ a , b , c ≤ 1 0
When a = 1 , c = 0 you have 10 values of c to choose from since it cant be 0 and 11 values of b to choose from
Which gives 1 0 ∗ 1 1 = 1 1 0 ways of picking c and b
So the total number of triples is
1 2 1 + 1 1 0 = 2 3 1
This post realizes me. Since I neglect the possibility that a = 1 :(
We can write a C++ Coding as follows;
void main()
{ clrscr();
int i,j,k,count=0,a,b;
for(i=0;i<=10;i++)
{ for(j=0;j<=10;j++)
{ for(k=0;k<=10;k++)
{
a = i*(j+k);
b = i*j+k;
if(a==b)
count++;
}
}
}
cout<<"\n\n\tAns = "<<count;
getch();
return;
}
Output: Ans = 231
Lets write the two expressions after distributing a .
Then we have:
a
×
b
+
a
×
c
=
=
a
×
b
+
c
Obviously there are only two cases when the two equations are equal:
When a = = 1
When c = = 0
a = = 1 O R c = = 0
When either of the two conditions are satisfied, both equations will be equal. Then for the 1st condition we have 1 1 × 1 1 possibilities, and for the 2nd one we also have 1 1 × 1 1 possibilities. Now some triplets have been counted twice and those are when c = 0 A N D a = 1 . By inclusion and exclusion principle, we need to subtract these triplets from our answer, so the final answer is ( 1 1 × 1 1 ) + ( 1 1 × 1 1 ) − ( 1 1 ) = 2 3 1
If we distribute a(b+c), we get ab+ac = ab + c. Then, we can subtract ab from both sides, ending with ac = c. Ordered pairs that satisfy this are ordered pairs with a = 1 or c = 0. Now we can do casework, counting the number of ordered pairs with (1,b,c) or (a,b,0). We have 11 possibilities for each letter, giving us 121 possibilities a = 1 and 121 for c = 1. However, we count any ordered pair (1,b,0) twice, which has 11 possibilities. Therefore, the grand total is 121+121-11=231.
If a x (b+c) = a x b + c, then a x c = c , then a = 1, * if c /= 0 * . We have: * if c /= 0 * the ordered triples are (1,b,c) with *c/= 0 * and so we have 11x10 = 110 triplets. If c = 0 we have axb = axb that is true for all a,b, so we have 11x11= 121 triplets like (a,b,0). In total we have 110+121=231 triplets.
Computer is always reliable for this kind of question. Answer: 231
we have two cases case 1. (when a=1) the possible number of triplets are 1 * 11 * 11=121 case 2. (when c=0) the possible number of triplets are ( 11 * 11 * 1)-(1 * 11 * 1)=110 we subtract because it has occurred in case 1. so the possible number of triplets which satisfy the property are 121+110=231
Did the same way !!!
counting using javascript
1 2 3 4 5 6 7 8 |
|
By expanding the equation, we get:
a × b + a × c = a × b + c
a × c = c ( 1 )
Now, the original equation will be true when (1) is true.
Since b is eliminated completely from the result equation, b can take any value in range [0,10] without affecting the validity of (1). There are 11 possibilities for b.
For (1) to be true, either:
a can only be 1. c can take any value in range (0,10], thus has 10 possibilities.
The number of combinations of a and c in this case is 1 × 1 0 = 1 0
a can take any value in range [0,10], thus has 11 possibilities. c can only be 0.
The number of combinations of a and c this time is 1 × 1 1 = 1 1
The number of total possible combination of a and c is 11 + 10 = 21.
The number of combinations of a,b and c are 1 1 × 2 1 = 2 3 1
that equation is correct if a=1, or c=0
number of orders with (a=1) = 11*11=121, as we have 11 numbers from 0 to 11 could be the values of b and c
number of orders with (c=0) = 11*11=121, as we have 11 numbers from 0 to 11 could be the values of a and b
number of orders with (a=1) and (c=0) = 11, as we have 11 numbers from 0 to 11 could be the values of b
number of orders with (a=1) or (c=0) =number of orders with (a=1)+number of orders with (c=0)-number of orders with (a=1) and (c=0)=121+121-11= 2 3 1
a(c-1)=0
Possibilities:
if a=1 then c can be from 0 to 10. if c=1 then a can be from 1 to 10. so totally 21 possibilities. b can be from 0 to 10, so 11 posibilities. 21*11=231
(axb)+(axc)=(axb)+c =>ac=c...this means it does not depend on b....so every 11 values of b is possible for each duplet (a,c)...so a factor of 11 is included to the number of triplets... now, for c not equal to 0,a=1 therefore, all duplets (a,c)= (1,1),(1,2)...(1,10)...that is 10 duplets are possible.... again,if c=0,all 11 values of a is valid...therefore 11 duplets so total number is=11x(11+10)=231
When c = 0, a and b both can take 11 values (0..10 inclusive) that gives us 11 x 11 possible triplets. When c = (1..10), a has to be 1 and b can still take 11 values. That gives us another 11 x 10 possible triplets. Add 121 + 110. That is 231.
Problem Loading...
Note Loading...
Set Loading...
Let's calculate what a × ( b + c ) actually is. By the distributive property,
a × ( b + c ) = a × b + a × c
Substituting a × b + a × c for a × ( b + c ) , we have the equation
a × b + a × c = a × b + c
We can subtract a × b from both sides of the equation to obtain a × c = c
Dividing c from both sides, we have a = 1 . However, c can also be equal to 0 (we can't divide by 0), since it ranges from 0 to 10 inclusive.
If a = 1 , there are 1 1 × 1 1 possibilities for b and c . Likewise, if c = 0 , there are 1 1 × 1 1 possibilities for a and b .
However, there are 1 1 possibilities we must subtract, since there are 1 1 possibilities for b when a = 1 and c = 0
Hence, we have 1 2 1 + 1 2 1 − 1 1 = 2 3 1 ■