Question
If $x = − 3,$ find the value of $2x^3 + 8x^2 – 15.$
✓
Answer
$x = − 3$
$\therefore 2x^3 + 8x^2 – 15$
$= 2 (– 3)^3 + 8 (– 3)^2 – 15$
$= 2 (– 27) + 8 (9) – 15$
$= – 54 + 72 – 15$
$= – 54 – 15 + 72$
$= – 69 + 72$
$= 3$
$\therefore 2x^3 + 8x^2 – 15$
$= 2 (– 3)^3 + 8 (– 3)^2 – 15$
$= 2 (– 27) + 8 (9) – 15$
$= – 54 + 72 – 15$
$= – 54 – 15 + 72$
$= – 69 + 72$
$= 3$
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