Try this beautiful problem from PRMO, 2012 based on Triangle.

Triangle | PRMO | Problem 12


In \(\triangle ABC\) we have \(AC=BC=7\) and \(AB=2\).Suppose that \(D\) is a point on line \(AB\) such that \(B\) lies between \(A\) and \(D\) and \(CD=8\) .what is the length of the segment \(DB\)?

  • \(5\)
  • \(3\)
  • \(7\)

Key Concepts


Geometry

Triangle

Pythagoras

Check the Answer


But try the problem first…

Answer:\(3\)

Source
Suggested Reading

PRMO-2012, Problem 7

Pre College Mathematics

Try with Hints


First hint

Problem based on Triangle

Given that \(AC=BC=7\) & \(CD=8\).we have to find out \(BD\).Let \(BD=x\).we draw a perpendicular from \(C\) to \(AB\) at the point \(M\).Therefore clearly \(\triangle CMB\) & \(\triangle CMD\) are right angle.so if we can find out the value of \(CM\) from the \(\triangle CMB\) then we can find out the value \(BD\) from the \(\triangle CMD\)

Can you now finish the problem ……….

Second Step

Problem based on Triangle

From the above diagram,In \(\triangle CMB\) we can say that \(CM=\sqrt{49-1}=4\sqrt 3\)

Given \(AB=2\) and \(M\) is the mid point of \(\triangle ABC\) (As AC=BC=7,Isosceles triangle),

Therefore \(BM=1\), So \(MD=x+1\)

Final Step

Figure of the Problem

From the \(\triangle CMD\), \((X+1)^2+(4\sqrt 3)^2=64\) \(\Rightarrow x=3,-5\)

we will take the positive value ,so \(BD=3\)

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