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Question Number 122636 by mnjuly1970 last updated on 18/Nov/20

...nice calculus... In AB^Δ C prove :: ∗: sin((A/2))sin((B/2))sin((C/2))≤(1/8) ......................... ∗∗:: max(cos((A/2))cos((B/2))cos((C/2)))=?

$$\:\:\:\:\:\:\:\:\:\:...{nice}\:\:{calculus}... \\ $$$$\:\:\:{In}\:\:\mathrm{A}\overset{\Delta} {\mathrm{B}C}\:\:{prove}\:::\: \\ $$$$\:\:\:\ast:\:\:{sin}\left(\frac{\mathrm{A}}{\mathrm{2}}\right){sin}\left(\frac{\mathrm{B}}{\mathrm{2}}\right){sin}\left(\frac{\mathrm{C}}{\mathrm{2}}\right)\leqslant\frac{\mathrm{1}}{\mathrm{8}} \\ $$$$......................... \\ $$$$\:\:\:\:\ast\ast::\:\:\:{max}\left({cos}\left(\frac{\mathrm{A}}{\mathrm{2}}\right){cos}\left(\frac{\mathrm{B}}{\mathrm{2}}\right){cos}\left(\frac{\mathrm{C}}{\mathrm{2}}\right)\right)=? \\ $$$$\:\:\:\:\: \\ $$$$\: \\ $$$$\:\:\:\:\:\:\: \\ $$

Answered by TANMAY PANACEA last updated on 18/Nov/20

three points A,B and C lie on curve y=sin(x/2) (A,sin(A/2)) (B,sin(B/2))and (C,sin(C/2)) centroid P (((A+B+C)/3),((sin(A/2)+sin(B/2)+sin(C/2))/3)) ordinate of centroid P=(((sin(A/2)+sin(B/2)+sin(C/2))/3)) pointQ={ (((A+B+C)/3))sin((((A+B+C)/3)/2))}lies on y=sin((x/2)) sin(((A+B+C)/6))=ordinate of pointQ sin(((A+B+C)/6))>((sin(A/2)+sin(B/2)+sin(C/2))/3) using AM>GM sin(((180^o )/6))>((sin(A/2)+sin(B/2)+sin(C/2))/3)>(sin(A/2)sin(B/2)sin(C/2))^(1/3) ((1/2))^3 >(sin(A/2)sin(B/2)sin(C/2)) proved

$$\:{three}\:{points}\:{A},{B}\:{and}\:{C}\:{lie}\:{on}\:{curve}\:{y}={sin}\frac{{x}}{\mathrm{2}} \\ $$$$\left({A},{sin}\frac{{A}}{\mathrm{2}}\right)\:\:\left({B},{sin}\frac{{B}}{\mathrm{2}}\right){and}\:\left({C},{sin}\frac{{C}}{\mathrm{2}}\right) \\ $$$${centroid}\:{P}\:\left(\frac{{A}+{B}+{C}}{\mathrm{3}},\frac{{sin}\frac{{A}}{\mathrm{2}}+{sin}\frac{{B}}{\mathrm{2}}+{sin}\frac{{C}}{\mathrm{2}}}{\mathrm{3}}\right) \\ $$$${ordinate}\:{of}\:{centroid}\:{P}=\left(\frac{{sin}\frac{{A}}{\mathrm{2}}+{sin}\frac{{B}}{\mathrm{2}}+{sin}\frac{{C}}{\mathrm{2}}}{\mathrm{3}}\right) \\ $$$${pointQ}=\left\{\:\left(\frac{{A}+{B}+{C}}{\mathrm{3}}\right){sin}\left(\frac{\frac{{A}+{B}+{C}}{\mathrm{3}}}{\mathrm{2}}\right)\right\}{lies} \\ $$$${on}\:{y}={sin}\left(\frac{{x}}{\mathrm{2}}\right) \\ $$$${sin}\left(\frac{{A}+{B}+{C}}{\mathrm{6}}\right)={ordinate}\:{of}\:{pointQ} \\ $$$${sin}\left(\frac{{A}+{B}+{C}}{\mathrm{6}}\right)>\frac{{sin}\frac{{A}}{\mathrm{2}}+{sin}\frac{{B}}{\mathrm{2}}+{sin}\frac{{C}}{\mathrm{2}}}{\mathrm{3}} \\ $$$${using}\:{AM}>{GM} \\ $$$${sin}\left(\frac{\mathrm{180}^{{o}} }{\mathrm{6}}\right)>\frac{{sin}\frac{{A}}{\mathrm{2}}+{sin}\frac{{B}}{\mathrm{2}}+{sin}\frac{{C}}{\mathrm{2}}}{\mathrm{3}}>\left({sin}\frac{{A}}{\mathrm{2}}{sin}\frac{{B}}{\mathrm{2}}{sin}\frac{{C}}{\mathrm{2}}\right)^{\frac{\mathrm{1}}{\mathrm{3}}} \\ $$$$\left(\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{3}} >\left({sin}\frac{{A}}{\mathrm{2}}{sin}\frac{{B}}{\mathrm{2}}{sin}\frac{{C}}{\mathrm{2}}\right) \\ $$$${proved} \\ $$$$ \\ $$$$ \\ $$$$ \\ $$$$ \\ $$$$ \\ $$

Commented by TANMAY PANACEA last updated on 18/Nov/20

same method for cosθ curve final step cos(((180^o )/6))>(cos(A/2)cos(B/2)cos(C/2))^(1/3) (((√3)/2))^3 >(cos(A/2)cos(B/2)cos(C/2)) ((3(√3))/2)>(cos(A/2)cos(B/2)cos(C/2))

$${same}\:{method}\:{for}\:{cos}\theta\:{curve} \\ $$$${final}\:{step} \\ $$$${cos}\left(\frac{\mathrm{180}^{{o}} }{\mathrm{6}}\right)>\left({cos}\frac{{A}}{\mathrm{2}}{cos}\frac{{B}}{\mathrm{2}}{cos}\frac{{C}}{\mathrm{2}}\right)^{\frac{\mathrm{1}}{\mathrm{3}}} \\ $$$$\left(\frac{\sqrt{\mathrm{3}}}{\mathrm{2}}\right)^{\mathrm{3}} >\left({cos}\frac{{A}}{\mathrm{2}}{cos}\frac{{B}}{\mathrm{2}}{cos}\frac{{C}}{\mathrm{2}}\right) \\ $$$$\frac{\mathrm{3}\sqrt{\mathrm{3}}}{\mathrm{2}}>\left({cos}\frac{{A}}{\mathrm{2}}{cos}\frac{{B}}{\mathrm{2}}{cos}\frac{{C}}{\mathrm{2}}\right) \\ $$

Commented by mnjuly1970 last updated on 18/Nov/20

thank you sir tanmay... excellent...

$${thank}\:{you}\:{sir}\:{tanmay}... \\ $$$$\:\:{excellent}... \\ $$

Commented by TANMAY PANACEA last updated on 18/Nov/20

most welcome

$${most}\:{welcome} \\ $$

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