If $y=2x+c$ is tangent to parabola $y^{2}=8x$ .Then find the value of $c$ .

The answer is 1.

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Tangent of slope $m$ to parabola $y^2=4(2)x$ is $y=mx +\dfrac{2}{m}$ . Tangent of slope $2$ will be : $y=2x+1$ $\implies c=\boxed 1$