∵
?
ex+e?x
2
?
ey+e?y
2
?
ex?e?x
2
=
ey?e?y
2
(ex+y+ex?y+e?x+y+e?x?y?ex+y+ex?y+e?x+y?e?x?y)=1 4
(2ex?y+2e?(x?y))＝1 4
＝ch(x?y)
ex?y+e?(x?y)
2
∴ch（x-y）=chx?chy-shx?shy
故答案为：ch（x-y）=chx?chy-shx?shy
（填入ch（x+y）=chx?chy+shx?shy，sh（x-y）=shx?chy-chx?shy，sh（x+y）=shx?chy+chx?shy也可）
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