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Question

If in an isosceles triangle with base a, vertical angle 20 and lateral side each of length b is given then value of a3+b3 equals
692774_7d44ef3709884688b34943e670ce5b7f.png

A
3ab
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B
3ab2
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C
3a2b
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D
3
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Solution

The correct option is B 3ab2
Given vertical angle=20o

B+B+20o=1802B=160 [Isosceles triangle ]

B=C=80o

Apply cosine formula at B :-

cos80o=a2+b2b22ab

cos80o=a2b

sin10o=a2b [ As cos80o=sin10o]

As we know that,

sin3θ=3sinθ4sin3θ

Put θ=10o

sin30o=3sin10o4sin310o

12=3×a2b4a×a38b3

1=3aba3b31=3ab2a3b3

3ab2a3=b3

a3+b3=3ab2

1446502_692774_ans_ae4fc6377afb4b4fa0c705039727a15c.png

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