已知tana=2
（1）(2cosa-√2sina)/(2cosa+√2sina)
=(2-√2tana)/(2+√2tana)
=(2-2√2)/(2+2√2)
=(1-√2)²/[(1+√2)(1-√2)]
=(1+2-2√2)/(1-2)
=-3+2√2
(2)3sin平方a-4sinacosa+cos平方a
=(3tan²a-4tana+1)/cos²a
=(3tan²a-4tana+1)*(1+tan²a)
=(3*4-4*2+1)(1+4)
=25