@suchi1996's post

in [2019] TIME AIMCAT 2019 (28nd June to 1st July)
Updated 2 months ago

one doubt

x^4-8X3+ax^2-bx+16=0 has +ve real roots

Find a-b

Now my doubt is

I applied AM>GM concept here

so a came out as 12 and b came out as 8

but since b is negative in the equation b should be -8??

@TanujA3 ans is 4

I am getting 20

@khan.moin00103

1 comment
khan.moin00103
@khan.moin00103IIM Calcutta  ·  560 karma

@suchi1996 Using AM>=GM, You will get all the 4 roots as 2.

a/1= Sum of product of all possible pairs.

so, a= (2*2)*6 = 24   (because 4C2=6)

-(-b)/1= Sum of product of all possible triplets.

So, b= (2*2*2)*4 = 32 .   (because 4C3=4)


So, a-b= -8.


I think the given answer is wrong. To check, you can just expand (x-2)^4, you will get x^4-8x^3+24x^2-32x+16

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