Roots of Cubic

Algebra Level 3

Given two roots of $x^3+ax+b=0$ are $2$ and $7$ .Then find $a+b-ab$ .

The answer is 8501.

1 solution

A Former Brilliant Member
Nov 2, 2015

Since two roots $2$ and $7$ of polynomial $x^3+ax+b=0$ are given.
Now putting value of $x$ .
● $8+2a+b=0$
$=2a+b=-8.....(1)$
● $343+7a+b=0$
$=7a+b=-343.....(2)$
Solving this two equations give
$a=\boxed{-67}$
$b=\boxed{126}$
Now,
$a+b-ab=(-67)+126-(-67×126)=\boxed{8501}$

Answer Should be 8501. (-67)+126-(126×(-67))=59+8442Gives us 8501.. Because There is minus sign Outside the bracket or Edit The problem By changing sign of ab from - ab to ab...

Akshay Sant - 5 years, 7 months ago

Roots of Cubic

The answer is 8501.

1 solution

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