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Find the values of `theta`, for which `cos3theta + sin3theta + (2sin2theta-3)(sintheta-costheta)` is always positive. |
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Answer» Given expression can be written as: `4cos^(3)theta-3costheta+3sintheta-4sin^(3)theta+(2sin2theta-3)(sintheta-costheta)` Applying given condtions, we get `rArr -4(sin^(3)theta-cos^(3)theta) + 3(sintheta-costheta) + (sintheta-costheta) (2sin2theta-3) gt 0` `rArr -4(sintheta-costheta)(sin^(2)theta + cos^(2)theta + sinthetacostheta)+3(sintheta-costheta) + (sintheta-costheta) (2sin2theta-3) gt 0` `rArr (sintheta-costheta) {-4 -4sintheta costheta+3+4sintheta-3} gt 0` `rArr -4(sintheta-costheta) gt 0` `rArr -4sqrt(2)sin(theta-pi/4) gt 0 rArr sin(theta-pi/4) lt 0 rArr 2npi - pi lt theta-pi/4 lt 2npi, n in I` `rArr 2npi - (3pi)/(4) lt theta lt 2npi + pi/4 rArr theta in (2npi -(3pi)/(4), 2npi + pi/4), n in I` Ans. |
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