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`[5cos theta+3cos(theta+pi/3)+3]` lies between (A) 4 and 10 (B) -4 and 10 (C) -10 and 4 (D) None of the above |
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Answer» `3cos(theta+pi/3)=3{costhetacospi/3-sinthetasinpi/3}` `=3costheta*1/2-3sinthetasqrt3/2` `=3/2costheta-3sqrt3/2sintheta` `5costheta+3cos(theta+pi/3)+3` `=5costheta+3/2costheta=3sqrt3/2sintheta+3` `=1/2(13costheta-3sqrt3sintheta)+3` `=sqrt(169+27)/2*cos(theta+phi)+3` `=sqrt196/2cos(theta+phi)+3` `=7cos(theta+phi)+3` `-4<=7cos(theta+phi)+3<=10` option c is correct. |
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