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**mark** determine the coordinates of the centre and the radius of the circle with equation: $\displaystyle 2x^2 + 2y^2 - 16x + 4y = 1$

i started by making it $\displaystyle 2[x^2 - 8x + (-4)^2] + 2[y^2 + 2y + (1)^2] = 1 + (-8)^2 + (2)^2$ and then

$\displaystyle 2(x - 4)^2 + 2(y + 1)^2 = \sqrt69$ and i said the coordinates are $\displaystyle (8, -1)$ and the radius $\displaystyle \sqrt69$

the book says the radius is $\displaystyle \frac {1}{2} \sqrt70$ and the coordinates are $\displaystyle (4, -1)$

can someone tell me how i'm wrong please? thankyou