$1.9\pm \sqrt{77} =\sqrt{11} \pm \sqrt{7} $
$3.T_{r+1}=\binom{n}{r} a^{n-r}b^r$
11.เพื่อให้ดูง่ายขึ้น
$\frac{a}{2-a} =-1+\frac{2}{2-a} $
$\frac{b}{6-b} =-1+\frac{6}{6-b} $
$\frac{c}{12-c} =-1+\frac{12}{12-c} $
$\therefore \frac{2}{2-a} =\frac{6}{6-b} =\frac{12}{12-c} $
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