Question 224302
A  rough  sketch  helps  to  visualize.
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the  base  of  the  isosceles  triangle is  21,  and  the  median  from  the  opposite  vertex  is  the  height,,,  which  is  9
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This  causes  a right  triangle, legs 9 and  10.5 (21/2)
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The  base  angle  of  the  triangle  has, tan x = opp/adj = h/(b/2) =9/10.5 = .8571
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the  angle  x = tan^-1 (.8571) = 40.60 degrees
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check....9/10.5 = tan 40.6,,,,.8571,,,,,ok