Question
In the figure, ABC; is triangle in which C = 90º and AB = 5 cm. D is a point on AB such that AD = 3 cm and = 60º. Then the length of AC is –
- 5cm
- cm
- cm
- none of these
Hint:
apply the sine rule to the triangle ACD and the m-n theorem to the triangle ACD.
The correct answer is: 5cm
5cm
Using the sine rule in triangle CDA, we get AC/sin CDA = AD/sin ACD
AC=3 x sin CDA /sin60
Using the m-n property, we can say that
(3+2) cot <CDA = 2 cot 30 – 3 cot 60
cot <CDA = √3/5
sin <CDA = 5/√28
AC = 3x 2 x 5/√3 x √28
AC = 5 √(3/7)
the m-n theorem states that
(m + n) cot θ = m cot α – n cot ß
this gives us the cotangent of the angle CDA which is further used in the sine rule to find the length of AC
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