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If A+B+c=180^0,\ then s e c A(cos B co...

If `A+B+c=180^0,\ ` then `s e c A(cos B cos C-sin B sin C)` is equal to `0` b. `-1` c. 1 d. none of these

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Verified by Experts

Given,
`A+B+C=pi`
=`B+C=pi−A`
`secA(cosBcosC−sinBsinC)`
=`secAcos(B+C)`
=`secAcos(pi−A)`
=`secA(−cosA)`
=`−1`
...
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