若sina的平方+sina=1,则cosa的平方+cosa的六次方+cosa的八次方等于?

cos^2a=1-sin^2a=sina
sin^2a=1-sina
cos^2a+cos^6a+cos^8a
=sina+sin^3a+sin^4a
=sina+sina(1-sina)+(1-sina)^2
=2sina-sin^2a+(1-2sina+sin^2a)
=1