Roots Progression

Algebra Level 3

If the equation

x 3 33 x 2 + 354 x k = 0 x^{3} - 33x^{2} + 354x - k = 0

has three roots and they are in an arithmetic progression.

Find the value of k.


The answer is 1232.

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1 solution

Let the three real roots be a , b a, b and c c .

From the topic of the "sum and product of roots",

we get :

a + b + c = 33 a + b + c = 33 ---------------------------(*)

a b + b c + c a = 354 ab + bc + ca = 354 --------------------(**)

a b c = k abc = k ---------------------------------------( * )

From equation (*):

Since a , b a, b and c c are in an arithmetic progression, let the common difference be n n .

So, ( b n ) + b + ( b + n ) = 33 (b - n) + b + (b + n) = 33

3 b = 33 3b = 33

b = 11 b = 11

Subsituting the value of b b into (**), we get:

11 ( a + c ) + a c = 354 11(a + c) + ac = 354

11 [ ( b n ) + ( b + n ) ] + a c = 354 11[(b - n) + (b + n)] + ac = 354

22 b + a c = 354 22b + ac = 354

242 + a c = 354 242 + ac = 354

a c = 112 ac = 112

From ( * ):

a b c = k abc = k

( 112 ) ( 11 ) = k (112)(11) = k

k = 1232 \boxed{k = 1232}

Sweet and simple

Chaitnya Shrivastava - 5 years, 7 months ago

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