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2015-04-17T20:51:00+05:30

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Since the triangle is isosceles, the other two sides have equal length. Call the length of each side x.

Draw a perpendicular from vertex to the base and recall that this is also a bisector of base. Therefore you get a right triangle with a leg 20 and one angle 70^{\circ}. The other side can be obtained by applying cosine ratio :
\cos(70^{\circ})~=~\frac{20}{x}\implies x = \frac{20}{\cos(70^{\circ})}~\approx~58.48

Add up the sides to get the perimeter
40+58.48+58.48~=~156.96
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thanks
can you answer my another question again ?