令 x =arccos (1/4),
则 x ∈(0,π),
且 cos x =1/4.
所以 sin x =√ [ 1 -(cos x)^2 ]
= √15 /4.
即 sin [ arccos (1/4) ] = √15 /4.
= = = = = = = = =
换元法.
注意 f(u) =arccos u 的值域.
arccos