Don't ask about the kangaroo

Algebra Level 1

If a , b a,b and c c are real numbers such that a b a\neq b and a 2 ( b + c ) = b 2 ( a + c ) = 2 , { a }^{ 2 }(b+c)={ b }^{ 2 }(a+c)= 2, find the value of c 2 ( a + b ) { c }^{ 2 }(a+b)


The answer is 2.

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3 solutions

0 = a 2 ( b + c ) b 2 ( a + c ) 0={ a }^{ 2 }(b+c)-{ b }^{ 2 }(a+c)

= a b ( a b ) + ( a 2 b 2 ) c =ab(a-b)+{ (a }^{ 2 }-{ b }^{ 2 })c

= ( a b ) ( a b + a c + b c ) =(a-b)(ab+ac+bc) but a b a\neq b ,

then a b + a c + b c = 0 ab+ac+bc =0 .

Multiplying by a c a-c

0 = ( a c ) ( a b + a c + b c ) 0=(a-c)(ab+ac+bc)

0 = a c ( a c ) + ( a 2 c 2 ) b 0=ac(a-c)+ ({ a }^2-{ c}^2)b

0 = a 2 ( b + c ) c 2 ( a + b ) 0=a^2(b+c)-{ c }^2(a+b)

then c 2 ( a + b ) = a 2 ( b + c ) = b 2 ( a + c ) = 2 { c }^2(a+b)= a^2(b+c)={ b }^{ 2 }(a+c)=\boxed{2}

Congratulations. A nice way of thinking.

Niranjan Khanderia - 6 years, 5 months ago

What's with that kangaroo? :D

Marc Vince Casimiro - 6 years, 5 months ago

I just substituted a = b = c = 1 a=b=c=1 . Btw why is that kangaroo?

Pranjal Jain - 6 years, 5 months ago

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It is given a is not equal to b.

Murugesh M - 6 years, 5 months ago

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Yeah... Let a = 1 a=1 and b= 1.00000....01 1.00000....01

It is just given to cancel off a b a-b as in @Angel Daniel Lopez Flores 's solution

Pranjal Jain - 6 years, 5 months ago

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@Pranjal Jain 1.00000....01 still has a limit of 1. Its like 0.99999...

Ryan McDaniel - 6 years, 5 months ago

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@Ryan McDaniel I mean 1 1 can be approximated as 1 + 1 0 9999999999 1+10^{-9999999999}

Pranjal Jain - 6 years, 5 months ago

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@Pranjal Jain I do not think that helps!

Niranjan Khanderia - 6 years, 5 months ago
Roman Frago
Jan 9, 2015

a 2 ( b + c ) = b 2 ( a + c ) = 2 a^2(b+c)=b^2(a+c)=2 a 2 ( b + c ) = b 2 ( a + c ) a^2(b+c)=b^2(a+c) c ( a 2 b 2 ) = a b ( b a ) c(a^2-b^2)=ab(b-a) c ( a + b ) ( a b ) = a b ( a b ) c(a+b)(a-b)=-ab(a-b) ( c ( a + b ) ) 2 = ( a b ) 2 (c(a+b))^2=(-ab)^2 c 2 ( a + b ) = a 2 b 2 a + b c^2(a+b)= \frac {a^2b^2} {a+b}

From the original equations: c = 2 a 2 b = 2 b 2 a c=\frac {2} {a^2}-b=\frac {2} {b^2}-a a b = 2 b 2 2 a 2 a-b=\frac {2} {b^2}-\frac {2} {a^2}

a b = 2 ( a + b ) ( a b ) a 2 b 2 a-b=2 \frac {(a+b)(a-b)} {a^2b^2} a 2 b 2 a + b = c 2 ( a + b ) = 2 \frac {a^2b^2} {a+b}=\boxed {c^2(a+b)=2}

Gamal Sultan
Jan 9, 2015

Since

(a^2)( b + c ) = (b^2)( a + c )

Then

( a - b )( a b + a c + b c ) = 0 , but a, b are not equal, then

a b + a c + b c = 0

c( a + b ) = - a b

Multiply by c

Then

(c^2)( a + b ) = - a b c ......................... (1)

Since

(a^2)( b + c ) = (b^2)( a + c ) = 2

Then

a + c = 2/b^2

b + c = 2/a^2

eliminating c

Then

(2/b^2) - a = (2/a^2) - b

Simplifying , then we get

a + b = (a^2)(b^2)/2

multiply by c^2

Then

(c^2)( a + b ) =(a^2)(b^2)(c^2)/2 ...............(2)

From (1) , (2) we get

a b c = -2

substituting in (1) we get

(c^2)( a + b ) = 2

really a nice problem

ahamed ibrahim - 6 years, 5 months ago

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