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Question

ABC, A′B′C′ are two triangles such that A+A′=1800, B=B′ Then

A
aa′=bb′+cc′
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B
ab′=ba′+cc′
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C
a′b=b′c+c′a
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D
bc′=b′c+ba′
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Solution

The correct option is A aa′=bb′+cc′
A+B+C=180o=A′+B′+C′
We have,
sinA′=sin(180o−A)=sinA and cosA′=cos(180o−A)=−cosA
Similarly, sinB′=sinB and cosB′=cosB

sin2A′sin2B′=sin2Asin2B
⇒sin2A(1−cos2B)=sin2B(1−cos2A)
⇒sin2A=sin2B+sin2Acos2B−cos2Asin2B
⇒sinAsinA′=sin2B+(sinAcosB+cosAsinB)(sinAcosB−cosAsinB)
⇒sinAsinA′=sinBsinB′+(sin(A+B))(sin(A−B))
⇒sinAsinA′=sinBsinB′+(sinC)sin(180o−A′−B′)
⇒sinAsinA′=sinBsinB′+sinCsinC′
Applying sine rule we get,
aa′=bb′+cc′

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