1.已知α是第三象限角,且tanα=3/4,则sinα/1-cotα + cosα/1-tanα等于 ( )A.7/5 B.7/4 C.-7/5 D.-7/4

1.已知α是第三象限角,且tanα=3/4,则sinα/1-cotα + cosα/1-tanα等于 ( )A.7/5 B.7/4 C.-7/5 D.-7/4
1.已知α是第三象限角,且tanα=3/4,则sinα/1-cotα + cosα/1-tanα等于 ( )
A.7/5 B.7/4 C.-7/5 D.-7/4

1.已知α是第三象限角,且tanα=3/4,则sinα/1-cotα + cosα/1-tanα等于 ( )A.7/5 B.7/4 C.-7/5 D.-7/4
1.已知α是第三象限角,且tanα=3/4,则sinα/(1-cotα) + cosα/(1-tanα)等于 ( )
A.7/5 B.7/4 C.-7/5 D.-7/4
∵α是第三象限角,且tanα=3/4,∴cotα=4/3,sinα=-3/5,cosα=-4/5
∴sinα/(1-cotα)+cosα/(1-tanα)=(-3/5)/(1-4/3)+(-4/5)/(1-3/4)=9/5-16/5=-7/5
故选C.

选C，因为a是第三象限角，所以sina<0.cosa<0,由tana=3/4可得，sina=-3/5,cosa=-4/5;原式可变形为-3*sina+4*cosa，将值带入后可知得-7/5.