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In trangle ABC Cos2A +cos2B+cos2C=cosB×cosC+cosC×cosA+cosA×cosBThen prove triangle are equilateral.
let cosA=x, cosB=y, cosC=zthe given qn then becomesx^2+y^2+z^2-xy-yz-zx=0or ((x-y)^2+(y-z)^2+(z-x)^2)/2=0this can only equal zero if x-y=y-z=z-x=0or, x=y=zor cosA=cosB=cosC
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