cos2pi9cospi18sin2pi9sinpi18SolutionWe have given cos2pi9cos

(cos2pi/9)(cospi/18)+(sin2pi/9)(sinpi/18)

We have given,

(cos2pi/9)(cospi/18)+(sin2pi/9)(sinpi/18)

we have a formula

cos(A-B) = cosA*cosB - sinA*sinB

Using this formula we have ,

(cos2pi/9)(cospi/18)+(sin2pi/9)(sinpi/18)

= cos (2pi/9 - pi/18)

= cos (3pi/18)

= cos(pi/6)

= (sqrt3)/2