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\paragraph{II.5}

$$xy=\left(x+y\over2\right)^2-\left(x-y\over2\right)^2$$

$$x={b\over2}+\sqrt{\left(b\over2\right)^2-c^2}\qquad
y={b\over2}-\sqrt{\left(b\over2\right)^2-c^2}$$

\paragraph{X.17}
$$x^2=bx-{a^2\over4}$$

$$x={b-\sqrt{b^2-a^2}\over2}$$

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