因为sinα=4/5
所以cosα=±√(1-sin²α)=±√(1-16/25)=±3/5
所以cosα=3/5或cosα=-3/5
所以tanα=sinα/cosα=4/5÷3/5=4/3
或tanα=4/5÷(-3/5)=-4/3
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