Question
Subtract $b(b^2+ b - 7) + 5$ from $3b^2- 8$ and find the value of expression obtained for $b = -3$.
✓
Answer
Required difference $= (3b^2- 8)[b(b^2+ b - 7) + 5]$
$ =3 b^2-8-b\left(b^2+b-7\right)-5 $
$ =3 b^2-8-b^3-b^2+7 b-5 $
$ =-b^3+2 b^2+7 b-13 $
Now, if $b = -3$
The value of expression $= -(-3)^2+ 2(-3)^2+ 7(-3) - 13$
$= -(-27) + 2 \times 9 - 21 - 13$
$= 27 + 18 - 21 - 13$
$= 45 - 34$
$= 11$
$ =3 b^2-8-b\left(b^2+b-7\right)-5 $
$ =3 b^2-8-b^3-b^2+7 b-5 $
$ =-b^3+2 b^2+7 b-13 $
Now, if $b = -3$
The value of expression $= -(-3)^2+ 2(-3)^2+ 7(-3) - 13$
$= -(-27) + 2 \times 9 - 21 - 13$
$= 27 + 18 - 21 - 13$
$= 45 - 34$
$= 11$
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