Originally Posted by

**mark** hi, my question is:

find the possible value of the constant c so that the line $\displaystyle y = 2x + c$ is a tangent to the circle $\displaystyle x^2 + y^2 = 4$

i treated it as a simultaneous equation (if thats the right thing to do) and continued by making equation one $\displaystyle y^2 = (2x + c)^2$ and subbed it into the second like this $\displaystyle x^2 + (2x + c)^2 = 4$ then expanded the second to $\displaystyle x^2 + 4x^2 + 4xc + c^2 = 4$ then simplified to $\displaystyle 5x^2 + 4xc + c^2 = 4$

i don't know what to do from here, can someone help me out please? thanks