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Question Number 37864 by kunal1234523 last updated on 18/Jun/18

prove that  cos (π/(15)) cos ((2π)/(15)) cos ((3π)/(15)) cos ((4π)/(15)) cos ((5π)/(15)) cos ((6π)/(15)) cos ((7π)/(15)) = (1/2^7 )

$$\mathrm{prove}\:\mathrm{that} \\ $$$$\mathrm{cos}\:\frac{\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{2}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{4}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{7}\pi}{\mathrm{15}}\:=\:\frac{\mathrm{1}}{\mathrm{2}^{\mathrm{7}} } \\ $$

Answered by tanmay.chaudhury50@gmail.com last updated on 18/Jun/18

cosx.cos2x.cos3x.cos4x.cos5x.cos6x.cos7x=k  x=(Π/(15))  2sinx.k=(2sinxcosx).cos2x.coz3x.coz4x.cos5x          cos6x.cos7x  2^2 sinx.k=(2sin2xcos2x)cos3xcos4xcos5xcos6x              cos7x  2^3 sinx.k=(2sin4xcos4x)cos3xcos5xcos6xco7x  2^3 sinx.k=(sin8x)cos3xcos5xcoz6xcos7x  now  15x=7x+8x  7x+8x=Π  8x=Π−7x  sin(8x)=sin(Π−7x)  sin8x=sin7x  2^4 sinx.k=(2sin7xcos7x)cos3xcos5xcos6x  2^4 sinx.k=sin14x.cos(3×(Π/(15)))(cos5×(Π/(15)))cos(6×(Π/(15))  15x=14x+x  Π=14x+x  14x=Π−x  sin(14x)=sin(Π−x)=sinx  2^4 .k=cos36^o ×cos60^o ×cos72^o   2^4 k=cos36^o ×(1/2)×sin18^o   2^5 k=sin18^o ×cos36^o   2^5 k=(((√5) −1)/4)×(((√5) +1)/4)  2^5 k=(4/(16))   2^5 ×2^2 ×k=1  k=(1/2^7 )

$${cosx}.{cos}\mathrm{2}{x}.{cos}\mathrm{3}{x}.{cos}\mathrm{4}{x}.{cos}\mathrm{5}{x}.{cos}\mathrm{6}{x}.{cos}\mathrm{7}{x}={k} \\ $$$${x}=\frac{\Pi}{\mathrm{15}} \\ $$$$\mathrm{2}{sinx}.{k}=\left(\mathrm{2}{sinxcosx}\right).{cos}\mathrm{2}{x}.{coz}\mathrm{3}{x}.{coz}\mathrm{4}{x}.{cos}\mathrm{5}{x} \\ $$$$\:\:\:\:\:\:\:\:{cos}\mathrm{6}{x}.{cos}\mathrm{7}{x} \\ $$$$\mathrm{2}^{\mathrm{2}} {sinx}.{k}=\left(\mathrm{2}{sin}\mathrm{2}{xcos}\mathrm{2}{x}\right){cos}\mathrm{3}{xcos}\mathrm{4}{xcos}\mathrm{5}{xcos}\mathrm{6}{x} \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:{cos}\mathrm{7}{x} \\ $$$$\mathrm{2}^{\mathrm{3}} {sinx}.{k}=\left(\mathrm{2}{sin}\mathrm{4}{xcos}\mathrm{4}{x}\right){cos}\mathrm{3}{xcos}\mathrm{5}{xcos}\mathrm{6}{xco}\mathrm{7}{x} \\ $$$$\mathrm{2}^{\mathrm{3}} {sinx}.{k}=\left({sin}\mathrm{8}{x}\right){cos}\mathrm{3}{xcos}\mathrm{5}{xcoz}\mathrm{6}{xcos}\mathrm{7}{x} \\ $$$${now}\:\:\mathrm{15}{x}=\mathrm{7}{x}+\mathrm{8}{x} \\ $$$$\mathrm{7}{x}+\mathrm{8}{x}=\Pi \\ $$$$\mathrm{8}{x}=\Pi−\mathrm{7}{x} \\ $$$${sin}\left(\mathrm{8}{x}\right)={sin}\left(\Pi−\mathrm{7}{x}\right) \\ $$$${sin}\mathrm{8}{x}={sin}\mathrm{7}{x} \\ $$$$\mathrm{2}^{\mathrm{4}} {sinx}.{k}=\left(\mathrm{2}{sin}\mathrm{7}{xcos}\mathrm{7}{x}\right){cos}\mathrm{3}{xcos}\mathrm{5}{xcos}\mathrm{6}{x} \\ $$$$\mathrm{2}^{\mathrm{4}} {sinx}.{k}={sin}\mathrm{14}{x}.{cos}\left(\mathrm{3}×\frac{\Pi}{\mathrm{15}}\right)\left({cos}\mathrm{5}×\frac{\Pi}{\mathrm{15}}\right){cos}\left(\mathrm{6}×\frac{\Pi}{\mathrm{15}}\right. \\ $$$$\mathrm{15}{x}=\mathrm{14}{x}+{x} \\ $$$$\Pi=\mathrm{14}{x}+{x} \\ $$$$\mathrm{14}{x}=\Pi−{x} \\ $$$${sin}\left(\mathrm{14}{x}\right)={sin}\left(\Pi−{x}\right)={sinx} \\ $$$$\mathrm{2}^{\mathrm{4}} .{k}={cos}\mathrm{36}^{{o}} ×{cos}\mathrm{60}^{{o}} ×{cos}\mathrm{72}^{{o}} \\ $$$$\mathrm{2}^{\mathrm{4}} {k}={cos}\mathrm{36}^{{o}} ×\frac{\mathrm{1}}{\mathrm{2}}×{sin}\mathrm{18}^{{o}} \\ $$$$\mathrm{2}^{\mathrm{5}} {k}={sin}\mathrm{18}^{{o}} ×{cos}\mathrm{36}^{{o}} \\ $$$$\mathrm{2}^{\mathrm{5}} {k}=\frac{\sqrt{\mathrm{5}}\:−\mathrm{1}}{\mathrm{4}}×\frac{\sqrt{\mathrm{5}}\:+\mathrm{1}}{\mathrm{4}} \\ $$$$\mathrm{2}^{\mathrm{5}} {k}=\frac{\mathrm{4}}{\mathrm{16}}\: \\ $$$$\mathrm{2}^{\mathrm{5}} ×\mathrm{2}^{\mathrm{2}} ×{k}=\mathrm{1} \\ $$$${k}=\frac{\mathrm{1}}{\mathrm{2}^{\mathrm{7}} } \\ $$

Answered by MrW3 last updated on 18/Jun/18

P=cos (π/(15)) cos ((2π)/(15)) cos ((3π)/(15)) cos ((4π)/(15)) cos ((5π)/(15)) cos ((6π)/(15)) cos ((7π)/(15))   2 sin (π/(15))P=2 sin (π/(15)) cos (π/(15)) cos ((2π)/(15)) cos ((3π)/(15)) cos ((4π)/(15)) cos ((5π)/(15)) cos ((6π)/(15)) cos ((7π)/(15))   2 sin (π/(15))P=sin ((2π)/(15)) cos ((2π)/(15)) cos ((3π)/(15)) cos ((4π)/(15)) cos ((5π)/(15)) cos ((6π)/(15)) cos ((7π)/(15))   2^2  sin (π/(15))P=2 sin ((2π)/(15)) cos ((2π)/(15)) cos ((3π)/(15)) cos ((4π)/(15)) cos ((5π)/(15)) cos ((6π)/(15)) cos ((7π)/(15))   2^2  sin (π/(15))P=sin ((4π)/(15)) cos ((3π)/(15)) cos ((4π)/(15)) cos ((5π)/(15)) cos ((6π)/(15)) cos ((7π)/(15))   2^3  sin (π/(15))P=2 sin ((4π)/(15)) cos ((3π)/(15)) cos ((4π)/(15)) cos ((5π)/(15)) cos ((6π)/(15)) cos ((7π)/(15))   2^3  sin (π/(15))P=sin ((8π)/(15)) cos ((3π)/(15)) cos ((5π)/(15)) cos ((6π)/(15)) cos ((7π)/(15))   2^4  sin (π/(15))P=2 sin ((8π)/(15)) cos ((3π)/(15)) cos ((5π)/(15)) cos ((6π)/(15)) cos ((7π)/(15))   2^4  sin (π/(15))P=2 sin ((7π)/(15)) cos ((3π)/(15)) cos ((5π)/(15)) cos ((6π)/(15)) cos ((7π)/(15))   2^4  sin (π/(15))P=sin ((14π)/(15)) cos ((3π)/(15)) cos ((5π)/(15)) cos ((6π)/(15))  2^4  sin (π/(15))P=sin (π/(15)) cos ((3π)/(15)) cos ((5π)/(15)) cos ((6π)/(15))  2^4  P=cos ((3π)/(15)) cos ((5π)/(15)) cos ((6π)/(15))  2^5  sin ((3π)/(15))P=2 sin ((3π)/(15)) cos ((3π)/(15)) cos ((5π)/(15)) cos ((6π)/(15))  2^5  sin ((3π)/(15))P=sin ((6π)/(15)) cos ((5π)/(15)) cos ((6π)/(15))  2^6  sin ((3π)/(15))P=2 sin ((6π)/(15)) cos ((5π)/(15)) cos ((6π)/(15))  2^6  sin ((3π)/(15))P=sin ((12π)/(15)) cos ((5π)/(15))  2^6  sin ((3π)/(15))P=sin ((3π)/(15)) cos ((5π)/(15))  2^6  P=cos ((5π)/(15))  2^6  P=cos (π/3)  2^6  P=(1/2)  ⇒P=(1/2^7 )

$${P}=\mathrm{cos}\:\frac{\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{2}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{4}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{7}\pi}{\mathrm{15}}\: \\ $$$$\mathrm{2}\:\mathrm{sin}\:\frac{\pi}{\mathrm{15}}{P}=\mathrm{2}\:\mathrm{sin}\:\frac{\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{2}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{4}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{7}\pi}{\mathrm{15}}\: \\ $$$$\mathrm{2}\:\mathrm{sin}\:\frac{\pi}{\mathrm{15}}{P}=\mathrm{sin}\:\frac{\mathrm{2}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{2}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{4}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{7}\pi}{\mathrm{15}}\: \\ $$$$\mathrm{2}^{\mathrm{2}} \:\mathrm{sin}\:\frac{\pi}{\mathrm{15}}{P}=\mathrm{2}\:\mathrm{sin}\:\frac{\mathrm{2}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{2}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{4}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{7}\pi}{\mathrm{15}}\: \\ $$$$\mathrm{2}^{\mathrm{2}} \:\mathrm{sin}\:\frac{\pi}{\mathrm{15}}{P}=\mathrm{sin}\:\frac{\mathrm{4}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{4}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{7}\pi}{\mathrm{15}}\: \\ $$$$\mathrm{2}^{\mathrm{3}} \:\mathrm{sin}\:\frac{\pi}{\mathrm{15}}{P}=\mathrm{2}\:\mathrm{sin}\:\frac{\mathrm{4}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{4}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{7}\pi}{\mathrm{15}}\: \\ $$$$\mathrm{2}^{\mathrm{3}} \:\mathrm{sin}\:\frac{\pi}{\mathrm{15}}{P}=\mathrm{sin}\:\frac{\mathrm{8}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{7}\pi}{\mathrm{15}}\: \\ $$$$\mathrm{2}^{\mathrm{4}} \:\mathrm{sin}\:\frac{\pi}{\mathrm{15}}{P}=\mathrm{2}\:\mathrm{sin}\:\frac{\mathrm{8}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{7}\pi}{\mathrm{15}}\: \\ $$$$\mathrm{2}^{\mathrm{4}} \:\mathrm{sin}\:\frac{\pi}{\mathrm{15}}{P}=\mathrm{2}\:\mathrm{sin}\:\frac{\mathrm{7}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{7}\pi}{\mathrm{15}}\: \\ $$$$\mathrm{2}^{\mathrm{4}} \:\mathrm{sin}\:\frac{\pi}{\mathrm{15}}{P}=\mathrm{sin}\:\frac{\mathrm{14}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}} \\ $$$$\mathrm{2}^{\mathrm{4}} \:\mathrm{sin}\:\frac{\pi}{\mathrm{15}}{P}=\mathrm{sin}\:\frac{\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}} \\ $$$$\mathrm{2}^{\mathrm{4}} \:{P}=\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}} \\ $$$$\mathrm{2}^{\mathrm{5}} \:\mathrm{sin}\:\frac{\mathrm{3}\pi}{\mathrm{15}}{P}=\mathrm{2}\:\mathrm{sin}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}} \\ $$$$\mathrm{2}^{\mathrm{5}} \:\mathrm{sin}\:\frac{\mathrm{3}\pi}{\mathrm{15}}{P}=\mathrm{sin}\:\frac{\mathrm{6}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}} \\ $$$$\mathrm{2}^{\mathrm{6}} \:\mathrm{sin}\:\frac{\mathrm{3}\pi}{\mathrm{15}}{P}=\mathrm{2}\:\mathrm{sin}\:\frac{\mathrm{6}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{6}\pi}{\mathrm{15}} \\ $$$$\mathrm{2}^{\mathrm{6}} \:\mathrm{sin}\:\frac{\mathrm{3}\pi}{\mathrm{15}}{P}=\mathrm{sin}\:\frac{\mathrm{12}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}} \\ $$$$\mathrm{2}^{\mathrm{6}} \:\mathrm{sin}\:\frac{\mathrm{3}\pi}{\mathrm{15}}{P}=\mathrm{sin}\:\frac{\mathrm{3}\pi}{\mathrm{15}}\:\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}} \\ $$$$\mathrm{2}^{\mathrm{6}} \:{P}=\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{15}} \\ $$$$\mathrm{2}^{\mathrm{6}} \:{P}=\mathrm{cos}\:\frac{\pi}{\mathrm{3}} \\ $$$$\mathrm{2}^{\mathrm{6}} \:{P}=\frac{\mathrm{1}}{\mathrm{2}} \\ $$$$\Rightarrow{P}=\frac{\mathrm{1}}{\mathrm{2}^{\mathrm{7}} } \\ $$

Commented by kunal1234523 last updated on 18/Jun/18

I appreciate your hard work

$$\mathrm{I}\:\mathrm{appreciate}\:\mathrm{your}\:\mathrm{hard}\:\mathrm{work} \\ $$

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