Entertaining Problem

This is a fun problem. May i request that only the students attempt this one?

If a,b and c are all distinct real numbers, then find values of a,b and c such that

\frac{a-b}{1+ab} + \frac{b-c}{1+bc} + \frac{c-a}{1+ca} = 0

21 Answers

11
Subash ·

so why is this particularly interesting or entertaining ?[7]

1
Akand ·

oh then we wud multiply by 32 and find d corresponding n2...??

62
Lokesh Verma ·

no i was just showing that you have to go from z=1 to z=2

so if you simply multiplied and divided by 4.. then the form would look like what it does here.....

now suppose the queston was

H-Li2+ mixture

Li is excited to the 2nd state. Then what would the answer be?

1
Akand ·

bhaiyya cud u plzzzzzzzz explain............y did u take n1 for He as 2 and n2 as 4?????
(n2=4 i kno u hav assumed it to make me understand but y n1 as 2)??

62
Lokesh Verma ·

R.12(1/12-1/22)

=

R22(1/22-1/42)

does this finally give you the correct answer?

1
Akand ·

k doubt in last year's paper....

plaese explain..............

1
Akand ·

ohhhhhhhh cool................................THANKS FOR ANOTHER TRICK..........................

62
Lokesh Verma ·

thi sis no twhat i meant ;)

well this surprising result:

tan A. tan 2A tan 3A= Tan 3A- Tan A - Tan 2A

1
Akand ·

tan2A=2tanA/1-tan2A
tan3A=tan(A+2A)=(tanA+tan2A)/(1-tanAtan2A)
=......expand tan2A here..............

but bhaiyya this is a normal formula naa...........
every1 knos these........

62
Lokesh Verma ·

Akand..

this is a very old problem

find the relation ship between

tan A, tan 2A and tan 3A

1
Akand ·

wow prophet sir...........tht method wud be awesum..........thnx for dis trick......ill remember it.........

1
Akand ·

wel a=b=c=0...??? but since they r distinct must think more hehe

341
Hari Shankar ·

@Akand One more useful fact is that

\tan (A+B+C) = \frac{\tan A + \tan B + \tan C - \tan A \tan B \tan C}{1- \tan A \tan B - \tan B \tan C - \tan C \tan A}

Here A+B+C = 0 and hence it follows that tan A + tan B + tan C = tan A tan B tan C

[you would have encountered a similar argument when A,B C are angles of a triangle]

See Loney for a general formula for

\tan(\theta_1 + \theta_2 +...+\theta_n)

1
Akand ·

yup bhaiyya..........................but who cud think like tht except rohan.........

k thnx anyways......nice trick though........TargetIIT is helpin me a lot....luv u guysssssss

62
Lokesh Verma ·

there is no formula..

just the fomula of tan (a+b) :P

1
Akand ·

is there a formula like that??????????????????????????

OMG.....................JEE is comin closer and im not prepared yet.........

I swear bhaiyya...............who cud think like that
tan(a+b)=tana+tanb/1-tanatanb
then cross multiplying and using here.............uff tooo much..........

waaaaahhhhhhhhhhhhhhhhhh..............

62
Lokesh Verma ·

first step is

tan (a) + tan b= tan (a+b)(1-tana tan b)

where a=x-y
b=y-z

1
Akand ·

rohan...............thnx a lot man............i was tryin this from tht time...........but i still cudnt get ur first step..........

62
Lokesh Verma ·

good lucid thinking :)

1
Rohan Ghosh ·

after akands step

just put

tan(x-y) + tan(y-z)=tan(x-z)(1-tan(x-y)tan(y-z))

it reduces to

tan(x-y)-tan(x-y)tan(x-z)tan(y-z)+tan(z-x)

=

tan(z-x)tan(x-y)tan(y-z)=0

either

tan(x-y)=0 or tan(y-z)=0 or tan(z-x)=0

from here we vget all solutions

1
Akand ·

wel i think we can take a as tanx b as tany and c as tanz

so it reduces to
tan(x-y)+tan(y-z)+tan(z-x)=0

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