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Question Number 57474 by malwaan last updated on 05/Apr/19

prove   sin18×cos36=(1/4)

$$\mathrm{prove}\: \\ $$$$\boldsymbol{{sin}}\mathrm{18}×\boldsymbol{{cos}}\mathrm{36}=\frac{\mathrm{1}}{\mathrm{4}} \\ $$

Commented by Abdo msup. last updated on 05/Apr/19

18→x  180→π ⇒180x =18π ⇒x =(π/(10)) ⇒36 =(π/5)  for that let prove sin((π/(10)))cos((π/5))=(1/4)  we have proved that cos((π/5))=((1+(√5))/4)  sin((π/(10)))=(√((1−cos((π/5)))/2))=(√((1−((1+(√5))/4))/2))  =(√((3−(√5))/8))=((√(3−(√5)))/(2(√2)))  we have { sin((π/(10)))cos((π/5))}^2   =((3−(√5))/8) ((6+2(√5))/(16)) =((3−(√5))/8) ((3+2(√5))/8) =((9−5)/(64)) =(4/(64)) =(1/(16)) ⇒  sin((π/(10))).cos((π/5)) =(1/4) .

$$\mathrm{18}\rightarrow{x} \\ $$$$\mathrm{180}\rightarrow\pi\:\Rightarrow\mathrm{180}{x}\:=\mathrm{18}\pi\:\Rightarrow{x}\:=\frac{\pi}{\mathrm{10}}\:\Rightarrow\mathrm{36}\:=\frac{\pi}{\mathrm{5}} \\ $$$${for}\:{that}\:{let}\:{prove}\:{sin}\left(\frac{\pi}{\mathrm{10}}\right){cos}\left(\frac{\pi}{\mathrm{5}}\right)=\frac{\mathrm{1}}{\mathrm{4}} \\ $$$${we}\:{have}\:{proved}\:{that}\:{cos}\left(\frac{\pi}{\mathrm{5}}\right)=\frac{\mathrm{1}+\sqrt{\mathrm{5}}}{\mathrm{4}} \\ $$$${sin}\left(\frac{\pi}{\mathrm{10}}\right)=\sqrt{\frac{\mathrm{1}−{cos}\left(\frac{\pi}{\mathrm{5}}\right)}{\mathrm{2}}}=\sqrt{\frac{\mathrm{1}−\frac{\mathrm{1}+\sqrt{\mathrm{5}}}{\mathrm{4}}}{\mathrm{2}}} \\ $$$$=\sqrt{\frac{\mathrm{3}−\sqrt{\mathrm{5}}}{\mathrm{8}}}=\frac{\sqrt{\mathrm{3}−\sqrt{\mathrm{5}}}}{\mathrm{2}\sqrt{\mathrm{2}}} \\ $$$${we}\:{have}\:\left\{\:{sin}\left(\frac{\pi}{\mathrm{10}}\right){cos}\left(\frac{\pi}{\mathrm{5}}\right)\right\}^{\mathrm{2}} \\ $$$$=\frac{\mathrm{3}−\sqrt{\mathrm{5}}}{\mathrm{8}}\:\frac{\mathrm{6}+\mathrm{2}\sqrt{\mathrm{5}}}{\mathrm{16}}\:=\frac{\mathrm{3}−\sqrt{\mathrm{5}}}{\mathrm{8}}\:\frac{\mathrm{3}+\mathrm{2}\sqrt{\mathrm{5}}}{\mathrm{8}}\:=\frac{\mathrm{9}−\mathrm{5}}{\mathrm{64}}\:=\frac{\mathrm{4}}{\mathrm{64}}\:=\frac{\mathrm{1}}{\mathrm{16}}\:\Rightarrow \\ $$$${sin}\left(\frac{\pi}{\mathrm{10}}\right).{cos}\left(\frac{\pi}{\mathrm{5}}\right)\:=\frac{\mathrm{1}}{\mathrm{4}}\:. \\ $$

Commented by malwaan last updated on 06/Apr/19

thank you sir

$$\boldsymbol{{thank}}\:\boldsymbol{{you}}\:\boldsymbol{{sir}} \\ $$

Answered by tanmay.chaudhury50@gmail.com last updated on 05/Apr/19

a=18^o   5a=2a+3a=90^o   2a=90^o −3a  sin2a=sin(90^o −3a)  2sinacosa=cos3a  2sinacosa=4cos^3 a−3cosa  cosa(2sina−4cos^2 a+3)=0  cosa≠0  2sina−4(1−sin^2 a)+3=0  4sin^2 a+2sina−1=0  sin18^o  >0  [ 18^o  lies in first quadrant]  sina=((−2+(√((2)^2 −4×4×(−1))))/(2×4))  sin18^o =((−2+2(√5))/8)  sin18^o =(((√5)  −1)/4)  cos36^o =1−2sin^2 18^o   =1−2((((√5) −1)/4))^2   =((16−2(5−2(√5) +1))/(16))=((8−(6−2(√5) ))/8)  =((2(√5) +2)/8)→(((√5) +1)/4)  sin18×cos36  =(((√5) −1)/4)×(((√5) +1)/4)  =((5−1)/(16))=(1/4)  i have derived the value of sin18^o  and cos36^o   then solved the problem..  for short cut method memorize the value  of sin18^o  and cos36^o  then solve it...

$${a}=\mathrm{18}^{{o}} \\ $$$$\mathrm{5}{a}=\mathrm{2}{a}+\mathrm{3}{a}=\mathrm{90}^{{o}} \\ $$$$\mathrm{2}{a}=\mathrm{90}^{{o}} −\mathrm{3}{a} \\ $$$${sin}\mathrm{2}{a}={sin}\left(\mathrm{90}^{{o}} −\mathrm{3}{a}\right) \\ $$$$\mathrm{2}{sinacosa}={cos}\mathrm{3}{a} \\ $$$$\mathrm{2}{sinacosa}=\mathrm{4}{cos}^{\mathrm{3}} {a}−\mathrm{3}{cosa} \\ $$$${cosa}\left(\mathrm{2}{sina}−\mathrm{4}{cos}^{\mathrm{2}} {a}+\mathrm{3}\right)=\mathrm{0} \\ $$$${cosa}\neq\mathrm{0} \\ $$$$\mathrm{2}{sina}−\mathrm{4}\left(\mathrm{1}−{sin}^{\mathrm{2}} {a}\right)+\mathrm{3}=\mathrm{0} \\ $$$$\mathrm{4}{sin}^{\mathrm{2}} {a}+\mathrm{2}{sina}−\mathrm{1}=\mathrm{0} \\ $$$${sin}\mathrm{18}^{{o}} \:>\mathrm{0}\:\:\left[\:\mathrm{18}^{{o}} \:{lies}\:{in}\:{first}\:{quadrant}\right] \\ $$$${sina}=\frac{−\mathrm{2}+\sqrt{\left(\mathrm{2}\right)^{\mathrm{2}} −\mathrm{4}×\mathrm{4}×\left(−\mathrm{1}\right)}}{\mathrm{2}×\mathrm{4}} \\ $$$${sin}\mathrm{18}^{{o}} =\frac{−\mathrm{2}+\mathrm{2}\sqrt{\mathrm{5}}}{\mathrm{8}} \\ $$$${sin}\mathrm{18}^{{o}} =\frac{\sqrt{\mathrm{5}}\:\:−\mathrm{1}}{\mathrm{4}} \\ $$$${cos}\mathrm{36}^{{o}} =\mathrm{1}−\mathrm{2}{sin}^{\mathrm{2}} \mathrm{18}^{{o}} \\ $$$$=\mathrm{1}−\mathrm{2}\left(\frac{\sqrt{\mathrm{5}}\:−\mathrm{1}}{\mathrm{4}}\right)^{\mathrm{2}} \\ $$$$=\frac{\mathrm{16}−\mathrm{2}\left(\mathrm{5}−\mathrm{2}\sqrt{\mathrm{5}}\:+\mathrm{1}\right)}{\mathrm{16}}=\frac{\mathrm{8}−\left(\mathrm{6}−\mathrm{2}\sqrt{\mathrm{5}}\:\right)}{\mathrm{8}} \\ $$$$=\frac{\mathrm{2}\sqrt{\mathrm{5}}\:+\mathrm{2}}{\mathrm{8}}\rightarrow\frac{\sqrt{\mathrm{5}}\:+\mathrm{1}}{\mathrm{4}} \\ $$$${sin}\mathrm{18}×{cos}\mathrm{36} \\ $$$$=\frac{\sqrt{\mathrm{5}}\:−\mathrm{1}}{\mathrm{4}}×\frac{\sqrt{\mathrm{5}}\:+\mathrm{1}}{\mathrm{4}} \\ $$$$=\frac{\mathrm{5}−\mathrm{1}}{\mathrm{16}}=\frac{\mathrm{1}}{\mathrm{4}} \\ $$$${i}\:{have}\:{derived}\:{the}\:{value}\:{of}\:{sin}\mathrm{18}^{{o}} \:{and}\:{cos}\mathrm{36}^{{o}} \\ $$$${then}\:{solved}\:{the}\:{problem}.. \\ $$$${for}\:{short}\:{cut}\:{method}\:{memorize}\:{the}\:{value} \\ $$$${of}\:{sin}\mathrm{18}^{{o}} \:{and}\:{cos}\mathrm{36}^{{o}} \:{then}\:{solve}\:{it}... \\ $$

Commented by malwaan last updated on 06/Apr/19

thank you so much

$$\boldsymbol{{thank}}\:\boldsymbol{{you}}\:\boldsymbol{{so}}\:\boldsymbol{{much}} \\ $$

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