arctan 0 = 0 //:tan 0 = 0,arctan 0 = 0
arctan 1 = π/4 //:tan (π/4) = 1 ,arctan 1 = π/4那arcsin、arccos 求法也是跟上面同理吗？arcsin (1/2) = π/6arccos (-1) = π注意反三角函数问题：求的是角度：arcsin(y)=?意思是：sin的甚么角度（x）等于y的意思。这样你就会求了。