`cos(-(9pi)/4)=`

Show that cos(pi/11)+cos((3pi)/11)+cos((5pi)/11)+cos((7pi)/11)+cos((9pi)/11)=1/2

show that cos^2(pi/9)+cos^2((2pi)/9)+cos^2((4pi)/9)=3/2

The number of roots of the equation sin(2x+(pi)/(18))cos(2x-(pi)/(9))=-(1)/(4) in [0,2 pi] is

Prove that 2cos(pi/13)cos((9pi)/13)+cos((3pi)/13)+cos((5pi)/13)=0

The value of cos^(-1){1/sqrt2(cos((9pi)/10)-sin((9pi)/10))}

The exact value of cos((2pi)/28)cosec((3pi)/28)+cos((6pi)/28)cosec((9pi)/28)+cos((18pi)/28)cosec((27pi)/28)

prove that : sin((5pi)/18)-cos((4pi)/9)=sqrt(3)sin(pi/9)

cos(pi/12)+sin^2((3pi)/12)+sin^2((9pi)/12)+cos((11pi)/12)=...

Prove that (1+"cos" (pi)/10)(1+"cos"(3pi)/10)(1+"cos"(7pi)/10)(1+"cos"(9pi)/10)=1/16 .