sin^2a+cos^2a=1...(1)sinacosa=2/5.(2)则cosa=2/(5sina)带入（1）得sin^2a+[2/(5sina)]^2=1有25sin^4a-25sin^2a+4=0(5sin^2a-4)(5sin^2a-1)=0解得sin^2a=4/5或1/5当sin^2a=4/5时cos^2a=1/5,当sin^2a=1/5时,cos^2a=4...