Rooting for the roots

Algebra Level 3

If a , b a,b are the roots of the equation x 2 10 c x 11 d = 0 x^2-10cx-11d=0 , and c , d c,d are the roots of the equation x 2 10 a x 11 b = 0 x^2-10ax-11b=0 , find the value of a + b + c + d a+b+c+d

Note: Here a c a ≠ c and b d b≠d


The answer is 1210.

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1 solution

Parth Sankhe
Nov 22, 2018

Applying sum of roots and product of roots,

a + b = 10 c ; c + d = 10 a a+b=10c; c+d=10a - (i)

a b = 11 d ; c d = 11 b ab=-11d; cd=-11b - (ii)

Adding both the equations in (i), we get a + b + c + d = 10 ( a + c ) b + d = 9 ( a + c ) a+b+c+d=10(a+c) \rightarrow b+d=9(a+c)

Equating the value of b d \frac {b}{d} from (ii), we get a c = 121 ac=121 . Now,

a 2 10 a c 11 d = 0 a^2-10ac-11d=0

c 2 10 a c 11 b = 0 c^2-10ac-11b=0

Adding both the equations above, we get,

( ( a + c ) 2 2 a c ) 20 a c 11 ( b + d ) = 0 ((a+c)^2-2ac)-20ac-11(b+d)=0

Putting a + c = t , b + d = 9 t , a c = 121 a+c=t, b+d=9t, ac=121 , we get a quadratic equation in t t .

( t 121 ) ( t + 22 ) = 0 (t-121)(t+22)=0 , t t = 121, -22.

But, if we make t = 22 t=-22 , that would mean a + c = 22 ; a c = 121 a = c = 11 a+c=-22;ac=121 \rightarrow a=c=-11 , but that is not allowed.

Hence, a + c = 121 a+c=121

a + b + c + d = 10 ( a + c ) = 1210 a+b+c+d=10(a+c)=1210

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