sin(π/4+α)=3/5
sinπ/4cosα + cosπ/4sinα = 3/5
根号2/2(cosα+sinα) = 3/5
等式两边平方得：
1/2(sin²α + 2sinαcosα + cos²α) = 9/25
2sinαcosα = 18/25 - 1
sin2α = -7/25