System of equations

Algebra Level 3

If the following system of equations is satisfied, find the product of all possible values of a a .

a 2 + a b + b 2 = 31 a^2+ab+b^2=31 a 2 a b + b 2 = 43 a^2-ab+b^2=43


The answer is 36.

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11 solutions

Anish Shah
Dec 14, 2013

Adding both the equation..

a 2 + b 2 = 37 a^{2} + b^{2} = 37

Subtracting both the equations...

a × b = 6 a \times b = -6

a = 6 b a = \frac{-6}{b}

Substitutiong b in the first equation,

a 2 + 36 a 2 = 37 a ^{2} + \frac{36}{a^{2}} = 37

Multiplying by a 2 a^{2} ,

a 4 37 a 2 + 36 = 0 a^{4} - 37a^{2} + 36 = 0

( a 2 36 ) ( a 2 1 ) = 0 (a^{2} - 36)(a^{2} - 1) = 0

a = 6 , 6 , 1 , 1 a = \boxed{6, -6, 1, -1}

Multiplying the solutions, we get the answer as \boxed{36}

after substituting b in the first equation and multiplying by a^2,it is a four degree equation in a and simply the product of roots is constant term divided by coefficientof a^4...............so no need to calculate individual roots

Kshitij Mishra - 7 years, 5 months ago

why cant the value of a be in decimal, or irrational, or even imaginary?

Sahil Gohan - 7 years, 1 month ago

Thats weird, why we need to multiply? Why the answer isn't 6?

Andrey Stephan - 7 years, 1 month ago

i followed the same method!!

Krishna Ramesh - 7 years, 1 month ago

Please mention that only integral values of 'a' are to be considered...

Samarpit Swain - 6 years, 11 months ago

Did it in the same manner to get the answer.

Ritu Roy - 6 years, 6 months ago
Will Song
Dec 14, 2013

Hello, we get a 2 + b 2 = 37 2 a b = 12 \begin{aligned} a^2 + b^2 &= 37 \\ 2ab &= 12 \end{aligned} so a + b = ± 5 a + b = \pm 5 .

a a then satisfies a 2 5 a 6 a^2 - 5a - 6 or a 2 + 5 a 6 a^2 + 5a - 6 and by Vieta we have ( 6 ) ( 6 ) = 36 (-6)(-6) = \boxed{36}

Solving the system of equations: Subtracting the first to the second equation gives 2ab = - 12 and ab = -6. This also implies that a^2 + b^2 = 37. Since ab = -6, then b = -6/a. a^2 + (36/a^2) = 37. Multiplying both sides by a^2 gives... a^4 + 36 = 37a^2 Solving for the roots gives a = 6, -6, 1, -1. Hence the product of the roots is 36.

Abubakarr Yillah
Jan 6, 2014

Let a 2 + a b + b = 31 . . . . . . . ( 1 ) {a}^2+{ab}+{b}={31} {.......(1)}

and a 2 + a b + b = 43 . . . . . . . ( 2 ) {a}^2+{ab}+{b}={43} {.......(2)}

from . . . . . . . ( 2 ) {.......(2)}

b 2 = 43 + a b a 2 {b}^2={43}+{ab}-{a}^2

substituting this into . . . . . . . ( 1 ) {.......(1)} we get

a 2 + a b + 43 + a b a 2 = 31 {a}^2+{ab}+{43}+{ab}-{a}^2={31}

from which we get 2 a b = 12 {2ab}={12}

a b = 6 {ab}={6}

the possible values of a are 3 , 2 , 6 a n d 1 {3},{2},{6} and {1}

and their product 3 × 2 × 6 × 1 {3}\times{2}\times{6}\times{1}

gives the answer of 36 \boxed{36}

sorry guys 2 a b = 12 {2ab}=-{12}

a b = 6 {ab}=-{6}

the possible values of a are 3 , 2 , 6 a n d 1 o r 3 , 2 , 6 a n d 1 {3},{2},{6} and {1} or {-3},{-2},{-6} and {-1}

Abubakarr Yillah - 7 years, 5 months ago
Fatin Farhan
Dec 14, 2013

a 2 + a b + b 2 = 31 , a 2 a b + b 2 = 43 a^2+ab+b^2=31, a^2-ab+b^2=43 . Adding and subtracting these two equation we get 2 a 2 + 2 b 2 = 74 < = > a 2 + b 2 = 37 2a^2+2b^2=74 <=> a^2+b^2=37 .......................(1)

2 a b = 12 2ab=-12 ................(2)

Now adding and subtracting the equations we get

( a + b ) 2 = 25 < = > ( a + b ) = ± 5 (a+b)^2=25 <=> (a+b)=\pm 5

( a b ) 2 = 49 < = > ( a b ) = ± 7 (a-b)^2=49 <=> (a-b)=\pm 7

Solving these equations we get a= 6 , 6 , 1 , 1 {6,-6,1,-1}

The product of all possible values of a is 36 \boxed{36}

Jeel Shah
Jul 6, 2014

Its weird that they don't give condition  that a & b are integers

Sahil Gohan
May 12, 2014

why cant the value of a be in decimal?

+6,-6,+1 and -1

John Palmer
Jan 8, 2014

subtract equations and you get 2ab=-12 = ab=-6. so a= -6, a=-1, a=1, a=6 work (note that if you check -2,3 and 2,-3 they do not work). so multiply the a solutions and you get 36! (note: that is not a factorial sign, it's only a normal exclamation point)

We have two term a^2 + ab + b^2 = 31 and a^2 - ab + b^2 = 43

adding all term we have 2a^2 + 2b^2 = 74

<=> 2(a^2 + b^2) = 74

<=> a^2 + b^2 = 37

  • a^2 + ab + b^2 = 31 then ab = -6

  • a^2 - ab + b^2 = 43 then - ab = 6

we assume a is positive, because factor positive of 6 is {1, 2, 3, 6} and this product is even

then the product of a = 1 x 2 x 3 x 6 = 36

Bro first of all a =2 doesn't satisfy the eqn.. And a can be -ve as well.. And a comes out to be ± 6 \pm 6 and ± 1 \pm1

Pankaj Joshi - 7 years, 5 months ago
Jiat Seng Teoh
Dec 16, 2013

a 2 + a b + b 2 = 31 e q ( 1 ) a^{2}+ab+b^{2}=31 ---------------------- eq(1)

a 2 a b + b 2 = 43 e q ( 2 ) a^{2}-ab+b^{2}=43 ----------------------- eq(2)

e q ( 1 ) e q ( 2 ) , eq(1)-eq(2),

2 a b = 12 2ab=-12

and a b = 6 ab=-6

S o , b = 6 a So, b= \frac{-6}{a}

e q ( 1 ) + e q ( 2 ) , eq(1)+eq(2),

2 ( a 2 + b 2 ) = 74 2(a^{2}+b^{2})=74

And, a 2 + b 2 = 37 a^{2}+b^{2}=37

a 2 + b 2 = 37 e q ( 3 ) a^{2}+b^{2}=37 ------------------- eq(3)

2 a b = 12 e q ( 4 ) 2ab=-12 -----------------------------eq(4)

e q ( 3 ) + e q ( 4 ) , eq(3)+eq(4),

a 2 + 2 a b + b 2 = 25 a^{2}+2ab+b^{2}=25

( a + b ) 2 = 25 (a+b)^{2}=25

( a + b ) = + 5 o r ( a + b ) = 5 (a+b)=+5 or (a+b)=-5

Since b = 6 a b= \frac{-6}{a} ,

a 6 a = + 5 a- \frac{6}{a} =+5 and a 6 a = 5 a- \frac{6}{a} = -5

a 2 6 = 5 a a^{2} - 6 = 5a and a 2 6 = 5 a a^{2} -6 = -5a

Solve the above equations and you'll get

a = 6 , 6 , 1 , a n d 1 a= 6, -6, 1, and -1

Hence, product of all possible value is 36

I did by same way

Ewerton Cassiano - 7 years, 5 months ago

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