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Let `alpha, beta, gamma` be the measures of angle such that `sin alpha+sin beta+sin gamma ge 2`. Then the possible value of `cos alpha + cos beta+cos gamma` isA. 3B. `2.5`C. `2.4`D. 2 |
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Answer» Correct Answer - D `sin alpha + sin beta + sin gamma ge 2` or `Sigma sin alpha ge 2` Squaring both sides, we get `Sigma sin^(2)alpha + 2 Sigma sin alpha sin beta ge 4` `rArr 3-Sigma cos^(2)alpha + 2Sigma sin alpha sin beta ge 4` `rArr 2 Sigma sin alpha sin beta ge 1 1+Sigma cos^(2) alpha` `rArr 2 Sigma (sin alpha sin beta + cos alpha cos beta) ge 1+(Sigma cos alpha)^(2)` (adding `2Sigma cos alpha cos beta` on both sides) `rArr (cos alpha + cos beta + cos gamma)^(2)+1le 2[cos (alpha-beta)+cos(beta-gamma)+cos(gamma-alpha)]le 6` `rArr (cos alpha + cos beta + cos gamma)^(2) le 5` `rArr (cos alpha + cos beta + cos gamma le sqrt(5)` |
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