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the question is http://home.earthlink.net/~urban-xrisis/phy.jpg [Broken]

[tex]g=\frac{GM}{x^2}[/tex]

[tex]x=\sqrt{r^2+a^2}[/tex]

[tex]g=\frac{GM}{(\sqrt{r^2+a^2})^2}[/tex]

[tex]g=\frac{GM}{r^2+a^2}[/tex]

since there are 2 masses...

[tex]g=2 \frac{GM}{r^2+a^2}[/tex]

my book's answer is:

[tex]g=\frac{2MGr}{(r^2+a^2)^{3/2}}[/tex]

what did I do wrong?

[tex]g=\frac{GM}{x^2}[/tex]

[tex]x=\sqrt{r^2+a^2}[/tex]

[tex]g=\frac{GM}{(\sqrt{r^2+a^2})^2}[/tex]

[tex]g=\frac{GM}{r^2+a^2}[/tex]

since there are 2 masses...

[tex]g=2 \frac{GM}{r^2+a^2}[/tex]

my book's answer is:

[tex]g=\frac{2MGr}{(r^2+a^2)^{3/2}}[/tex]

what did I do wrong?

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