关键是饭三角函数的值域
arcsin[-π/2,π/2]
arccos[0,π]
arctan[-π/2,π/2]
arccot[0,π]
arctan(tan(π/2-2))
-π/2<π/2-2<π/2,在函数定义域内
=π/2-2
x =1/2arccos4/5
x属于[0,π/2]
cos2x = 4/5 = (1-(tanx)^2)/(1+(tanx)^2)
=>
tanx=1/3
=>
tan(1/2arccos4/5)=1/3
x=arxsin(-3/5)
x属于[-π/2,π/2]
cos(arcsin(-3/5))
=cosx
=4/5
arcsin(cos1/3)
arcsin(sin(π/2-1/3))
=π/2-1/3
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