f(x)=sinx+cosx
=√2[sinx*(√2/2)+cosx(√2/2)]
=√2[sinxcos(π/4)+cosxsin(π/4)]
=√2sin(x+π/4)
sinα+cosα=3/4
平方得1+2sinαcosα=9/16
所以1+sin2α=9/16
所以sin2α=-7/16
f(x)=sinx+cosx
=√2[sinx*(√2/2)+cosx(√2/2)]
=√2[sinxcos(π/4)+cosxsin(π/4)]
=√2sin(x+π/4)
sinα+cosα=3/4
平方得1+2sinαcosα=9/16
所以1+sin2α=9/16
所以sin2α=-7/16