Prove that |x+y+z|<=|x|+|y|+|z|
let a,b∈R\text {let }a,b\in{R}let a,b∈R
a∗b≤∣a∣∗∣b∣a*b \le|a|*|b|a∗b≤∣a∣∗∣b∣
(a+b)2=a2+b2+2a∗b≤a2+b2+2∣a∣∗∣b∣(a+b)^2=a^2+b^2+2a*b\le{a^2+b^2+2|a|*|b|}(a+b)2=a2+b2+2a∗b≤a2+b2+2∣a∣∗∣b∣
a2+b2+2∣a∣∗∣b∣=∣a∣2+∣b∣2+2∣a∣∗∣b∣=(∣a∣+∣b∣)2a^2+b^2+2|a|*|b|=|a|^2+|b|^2+2|a|*|b|=(|a|+|b|)^2a2+b2+2∣a∣∗∣b∣=∣a∣2+∣b∣2+2∣a∣∗∣b∣=(∣a∣+∣b∣)2
(a+b)2≤(∣a∣+∣b∣)2(a+b)^2\le(|a|+|b|)^2(a+b)2≤(∣a∣+∣b∣)2
(a+b)2≤(∣a∣+∣b∣)2\sqrt{(a+b)^2}\le\sqrt{(|a|+|b|)^2}(a+b)2≤(∣a∣+∣b∣)2
∣a+b∣≤∣a∣+∣b∣ (1)|a+b|\le|a|+|b|\ (1)∣a+b∣≤∣a∣+∣b∣ (1)
∣x+y+z∣ let a=x+y;b=z|x+y+z| \text{ let }a=x+y;b=z∣x+y+z∣ let a=x+y;b=z
from inequality (1)\text{from inequality }(1)from inequality (1)
∣x+y+z∣≤∣x+y∣+∣z∣≤∣x∣+∣y∣+∣z∣|x+y+z|\le|x+y|+|z|\le|x|+|y|+|z|∣x+y+z∣≤∣x+y∣+∣z∣≤∣x∣+∣y∣+∣z∣
proved.
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