A curve has the equation y=2x^2-6x+c, where c is a constant. Find the value of c for which the line y+2x=8 is a tangent to the curve
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Rewrite your tangent as $\displaystyle y = -2x + 8$. The gradient of this tangent is $\displaystyle -2$. So this means $\displaystyle \frac{dy}{dx} = -2$. Find the derivative, set it equal to $\displaystyle -2$ and solve for $\displaystyle c$.
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