Mistakes give rise to problems- 2

Algebra Level 4

a + b c = a c + b c \dfrac{a+b}{c} = \dfrac{a}{c} + \dfrac{b}{c}

We know that we can split a fraction like what we did above, but you can't do this in

a b + c = a b + a c \dfrac{a}{b+c} = \dfrac{a}{b} + \dfrac{a}{c}

This is a false property, it will be a mistake if you do it! However, it holds for certain values of integers a , b , a,b, and c . c. If a , b , c 10 a,b,c \leq 10 and a , b , c a,b,c are non-negative integers, then for how many ordered triples ( a , b , c ) (a,b,c) is a b + c = a b + a c \frac a{b+c} = \frac ab + \frac ac seen to be true?


This problem is a part of the set Mistakes give rise to Problems . Try other problems of the set too.


The answer is 100.

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10 solutions

Aditya Raut
Jul 21, 2014

case 1 is a 0 a\neq 0 .

If a 0 a\neq 0 , we have to find the triples such that 1 b + c = 1 b + 1 c \dfrac{1}{b+c} = \dfrac{1}{b} + \dfrac{1}{c} and in 0 b , c 0 \leq b,c , always 1 b + c < 1 b \dfrac{1}{b+c} < \dfrac{1}{b} because the case of c = 0 c=0 makes 1 c \dfrac{1}{c} not defined .

Hence this gives us that case 2 , i.e. a = 0 a=0 to be the only case.

For a = 0 a=0 , b b and c c can be any pair of integers from 1 1 to 10 10 , hence there are 10 10 choices each for b b and c c giving 10 × 10 = 100 10\times 10 = 100 distinct ordered pairs ( b , c ) (b,c) , and adding a 0 0 to each of them in the start will give ( a , b , c ) (a,b,c) , hence there will be 100 \boxed{100} triples following the property.

Damn ! I misunderstood ordered pairs for a<b<c. I'm such a fool.

Keshav Tiwari - 6 years, 10 months ago

Shouldn't answer be 121 because a,b,c are nonnegative

Pranav Kirsur - 6 years, 5 months ago

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But b, c cannot be zero.... This will make the expression undefined.

Inderjeet Nair - 5 years, 4 months ago

I think the answer should be 90. There are 10 cases where b=c, thus making the triplet (a,b,c) the same as (a,c,b).

Ashwin Tare - 5 years, 10 months ago

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The question asks for the number of ordered triples. So each triple will be considered unique.

Rishik Jain - 5 years, 5 months ago

Did the same way.!!

Bhupendra Jangir - 5 years ago

Don't you think the cases when b = c b=c should be removed once as ( 0 , b , c ) ({0,b,c}) will be same as ( 0 , c , b ) ({0,c,b}) ?

Vishal Yadav - 5 years, 6 months ago

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Try writing the ordered pairs one by one, you will get the answer 100. Let me explain you. The ordered pairs can be divided into 10 groups depending upon the value of b, like (0,1,c),(0,2,c),(0,3,c),....(0,10,c). Now for each of the groups put different values of c from 1 to 10. Notice that due to different values of b the special case of b=c arises only once. So this implies that we have not recounted anything. Hence the answer 100.

Puneet Pinku - 5 years ago

I think its wrong ...correct answer is 90...because if you take n numbers and you want to create a set of 2 numbers such that two numbers are not equal then you can make n(n-1)sets...for example if you take numbers 1,2,3 and you want to make a set of two numbers then possibilities are... (1,2), (1,3),(2,1), (2,3),(3,1), (3,2) and we have to exclude (1,1),(2,2) and (3,3) as you have mentioned that they are "ordered triples".....so total no. of sets you can make are six that is 3(3-1) which is equal to six....and not 3×3=9 ...hence dude I think this answer is wrong...

Rahul Shete - 6 years, 8 months ago

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Distinct ordered pairs simply means that ordered pairs ( b , c ) (b,c) and ( c , b ) (c,b) are to be considered distinct pairs and not the same .

It doesn't mean that ( b , b ) (b,b) or ( c , c ) (c,c) are not distinct.

So the answer is right.

Gaurang Pansare - 6 years, 7 months ago

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No, once you set the values of b and c there is no way to distinguish between the ordered triples. For example, if b=c=1, what looks like distinct triples (a,b,c) and (a,c,b) turns into (0,1,1) and (0,1,1) so they're not distinct anymore

kimi p - 5 years, 4 months ago

Try writing the ordered pairs one by one, you will get the answer 100. Let me explain you. The ordered pairs can be divided into 10 groups depending upon the value of b, like (0,1,c),(0,2,c),(0,3,c),....(0,10,c). Now for each of the groups put different values of c from 1 to 10. Notice that due to different values of b the special case of b=c arises only once. So this implies that we have not recounted anything. Hence the answer 100.

Puneet Pinku - 5 years ago
Qi Huan Tan
Aug 7, 2014

Another method to prove that there is no solution when a 0 a\neq0 :

1 b + c = 1 b + 1 c \frac{1}{b+c}=\frac{1}{b}+\frac{1}{c} implies that b c = ( b + c ) 2 bc=(b+c)^2 or b 2 + c 2 = b c b^2+c^2=-bc . Moreover, b 2 + c 2 = b c 0 b^2+c^2=-bc\leq0 . Hence, b = c = 0 b=c=0 which contradicts the given equation.

Or can I do this:

(b+c)^2 = bc

-> b^2 + 2bc +c^2 = bc

-> b^2+ bc + c^2 = 0

-> 2(b^2) +2bc + c^2 = 0

-> (b+c)^2 + b^2 +c^2 = 0

-> b = c = 0 (contradicts with the given equation)

Huân Lê Quang - 5 years, 10 months ago

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Ya, you can. There is a typo in the third last row, it should be 2 b 2 + 2 b c + 2 c 2 = 0 2b^2+2bc+2c^2=0

Qi Huan Tan - 5 years, 10 months ago

I am a beginner that's why I may be wrong but I didn't think that it is given anywhere in question that a cannot be 0. In question it is stated that a is non-negative integer and as per my knowledge 0 is neither negative nor positive, then why all of you guys are stating that a is not equal to 0. Please help!!

Mayank Prakash - 6 years, 8 months ago

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He is not saying that 'a' is not equal to zero.... he has supposed that and has proved that it will be wrong and there will be no possibility satisfying the sum....hence a should be equal to 0...

Rahul Shete - 6 years, 8 months ago

( 1 ) b 2 + c 2 = b c . S i n c e b 0 a n d c 0 , t h i s i s n o t p o s s i b l e . Hove ever if both the sides are 0, then only this is possible. This could happen only if a=0. And then b and c take any value, except 0. So a=0 only and both b, c, 1 to 10, gives, 1 10 10 = 100 O R D E R E D T r i p l e s . (1)~\implies~b^2+c^2=- b*c. ~Since~b\nless 0 ~and~c\nless 0,~~this~ is ~not~ possible.\\ \text{Hove ever if both the sides are 0, then only this is possible.}\\ \text{This could happen only if a=0. And then b and c take any value, except 0.}\\ \text{So a=0 only and both b, c, 1 to 10, gives, }\\ 1*10*10=\color{#D61F06}{100}~ ~ORDERED~Triples.

Jesse Nieminen
Aug 22, 2015

We have 2 cases.

Case 1: a = 0 \text{Case 1: } {a = 0} In this case the equation is true for any values of b and c excluding 0 \text{In this case the equation is true for any values of b and c excluding 0} Therefore we have 100 solutions for case 1. \text{Therefore we have 100 solutions for case 1.}

Case 2: a 0 \text{Case 2: } {a \neq 0} a b + c = a b + a c {\frac{a}{b + c} = {\frac{a}{b} + \frac{a}{c} } } 1 b + c = 1 b + 1 c \Rightarrow {\frac{1}{b + c} = {\frac{1}{b} + \frac{1}{c} } } ( b + c ) 2 = b c \Rightarrow { {\left(b+c\right)}^{2} = {bc}} b 2 + b c + c 2 = 0 \Rightarrow {{b}^{2} + bc + {c}^{2} = 0} Which has no solutions when b > 0 , c > 0 \text{Which has no solutions when } {b > 0, c > 0}

Therefore the answer is 100 \boxed{100}

Moderator note:

Good usage of the cases to prevent a division by 0.

Case 1 : if a = 0 Then the given equation is true for any value of b and c in the given range, therefore triplets formed are 1×10×10 = 100 (b,c≠0). Moreover if a≠0, then formed condition is (b+c)^2 = bc which is only true if b,c=0 but this these values are neglected since it would render the given equation indeterminable. Thus total number of triplets is 100.

Thomas Fry
Apr 27, 2016

You're wrong suggesting it's 90, not because (0,b,b) and (0,c,c) are different, but because in counting to 100 they are not counted twice. (0,1,1...10) (0,2,1...10) (0,3,1...10)... See? The only bracket in my answer that leads to (0,1,1) is the first bracket and it only appears once. So 1x10x10 is suitable for receiving the correct answer of 100. Its good to spot that (a=0,b=1,c=1) only appears once so the ten cases where b=c do not appear twice and needn't be removed :)

Sridhar Sri
Mar 5, 2016

A small doubt what if we multiply both sides by ( b c ) (b-c) when b 2 + c 2 + b c = 0 b^2+c^2+bc=0 which leads us to b 3 c 3 = 0 b^3-c^3=0 thus b = c b=c which leads to another set of solutions. Please correct me where if I am wrong 😆.

Chaitnya Shrivastava - 5 years, 3 months ago
Lu Chee Ket
Oct 5, 2015

Just be careful to preserve the original equation. (0, b, c) for 1 to 10 of b and c gives 10 x 10 = 100.

1 = 1 + 1 + c/ b + b/ c is impossible to be true for all positive numbers.

Answer: 100

Sakshi Rathore
Sep 6, 2015

if there was written unordered pairs then what is the difference in both of its answers

In that case, we will have to delete some solutions, as then 0,9,8 will be same as 0,8,9.

Abhijeet Verma - 5 years, 8 months ago
Yash Singh
Jul 9, 2015

did the same way as Qi Huan Tan. this question could also be done by logic . we know from the question that the property is wrong and is supported(dependent) on the the denominator and hence it should mean for any value of "b" and "c" it is not true unless the value of the fraction is irrespective the value of "b" and "c" under the defined domain of "b"and "c" in the question . and the value of the fraction becomes independent of the "b"and "c" if "a=0". and then it is just the combination of a 10x10 square. =100

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