1. ## Equation

(3x +a) (bx-3)= 6x^2 +x-15

The equation is true for all values of x, when a and b are constant. What is the value of ab?

i did the foil method and came up with 10. Is that correct? or is it -10?

2. ## Re: Equation

+10 is correct

3. ## Re: Equation

Originally Posted by tahirakw
(3x +a) (bx-3)= 6x^2 +x-15

The equation is true for all values of x, when a and b are constant. What is the value of ab?

i did the foil method and came up with 10. Is that correct? or is it -10?
\displaystyle \begin{align*} (3x + a)(b\,x - 3) &= 3b\,x^2 + (ab - 9)x - 3a \end{align*}

For this to be equal to \displaystyle \begin{align*} 6x^2 + x - 15 \end{align*} it requires that \displaystyle \begin{align*} ab - 9 = 1 \end{align*}, and so \displaystyle \begin{align*} ab = 10 \end{align*} as you found.