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nonagerian

· started a discussion

· 1 Months ago

this is not good (Question: 111644.0 )

yeh toh cosine rule 10 th ke bad iit ki coaching me padhaya jata hai

Question:
The adjacent sides of a parallelogram are 2a and a. If the angle between sides of parallelogram is 60°, then one of the diagonals of the parallelogram is:
Options:
A) 3a 
B) 2a
C) \(\sqrt[]{7}a\)
D) \(\sqrt[]{5}a\)
Solution:
Ans: (c)


Clearly


\(\angle \)ABC = 120°

From \(\triangle\)CAB,

cos B = \(\cfrac{C^2+a^2-b^2}{2ca}\)

we get

cos (180o- 60o) =\(\cfrac{4a^2+a^2-AC^2}{2\times2a\times a}\)

or \(-\cfrac{1}{2}=\cfrac{5a^2-AC^2}{4a^2}\)

or -2a2 = 5a2 - AC2

or AC2 =7a2

\(\therefore AC =\sqrt{7}a\)

Knowledge Expert

· commented

· 1 Months ago

Dear Student, We have not taken the angle as 90 , please see the image carefully. Angle between the sides is 60 and the angle between side 2a and diagnol is 30. Given answer is right. Thanks and Regards Team TR
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