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Three vertices are chosen randomly from the seven vertices of a regular 7-sided polygon. The probability that they form the vertices of an isosceles triangle is-
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$\frac{3}{5}$
$\Delta \mathrm{AGB}, \triangle \mathrm{AFC} \& \triangle \mathrm{AED}$ are isosceles
$$
P=\frac{{ }^{7} \mathrm{C}_{1} \times 3}{{ }^{7} \mathrm{C}_{3}}=\frac{7 \times 3}{\frac{7 \times 6 \times 5}{3 \times 2}}=\frac{3}{5}
$$
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