f(x)有界即|存在一个正数M,使得在定义域内f(x)都满足
|f(x)|≤M,即
-M≤f(x)≤M
即f(x)上界为M,下界为-M
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当f(x)有上界M1,下界M2时,即M2≤f(x)≤M1,
取M=max{|M1|,|M2|},则必有-M≤f(x)≤M
即|f(x)|≤M
∴f(x)有界
命题得证
f(x)有界即|存在一个正数M,使得在定义域内f(x)都满足
|f(x)|≤M,即
-M≤f(x)≤M
即f(x)上界为M,下界为-M
-----------------------
当f(x)有上界M1,下界M2时,即M2≤f(x)≤M1,
取M=max{|M1|,|M2|},则必有-M≤f(x)≤M
即|f(x)|≤M
∴f(x)有界
命题得证