Math Prove That,cosπ7−cos2π7+cos3π7=12 September 26, 2023 Manish Verma Leave a comment L.H.S.=cosπ7–cos2π7+cos3π7 = cosπ7+cos3π7–cos(π–5π7) = cosπ7+cos3π7+cos5π7 = cos(π7+3–12.2π7).sin(3.2π/72)sin(2π/72) = cos(π7+2π7).sin3π7sinπ7 = cos3π7sin3π7sinπ7=2cos3π7sin3π72sinπ7 = sin6π72sinπ7=sin(π–π7)2sinπ7=12 Share this: Click to share on X (Opens in new window) X Click to share on Facebook (Opens in new window) Facebook Related