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anemone
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Prove that if $a,\,b,\,c,\,d>0$ and $a\le 1,\,a+b\le 5,\,a+b+c\le 14,\,a+b+c+d\le 30$, then $\sqrt{a}+\sqrt{b}+\sqrt{c}+\sqrt{d}\le 10$.
lfdahl said:Did I miss something, or is this problem really quite easy to solve? Thankyou for any comment!
Given the conditions:
$a \leq 1 \;\;\wedge \;\;a+b \leq 5\;\; \wedge \;\;a+b+c\leq 14\;\; \wedge \;\; a+b+c+d \leq 30$
which by successive subtractions implies:
$a \leq 1 \;\;\wedge \;\;b \leq 4\;\; \wedge \;\;c\leq 9\;\; \wedge \;\; d \leq 16$
or:
$\sqrt{a} \leq 1 \;\;\wedge \;\;\sqrt{b} \leq 2\;\; \wedge \;\;\sqrt{c}\leq 3\;\; \wedge \;\; \sqrt{d} \leq 4$
Adding the four inequalities yields:
$\sqrt{a} +\sqrt{b} +\sqrt{c}+\sqrt{d} \leq 1+2+3+4 = 10.$
anemone said:Prove that if $a,\,b,\,c,\,d>0$ and $a\le 1,\,a+b\le 5,\,a+b+c\le 14,\,a+b+c+d\le 30$, then $\sqrt{a}+\sqrt{b}+\sqrt{c}+\sqrt{d}\le 10$.
The inequality means that the sum of the square roots of positive numbers a, b, c, and d is less than or equal to 10.
There are multiple ways to prove this inequality, but one approach is to use the Cauchy-Schwarz inequality. This states that for any positive numbers x and y, we have $(x+y)^2 \le 2(x^2+y^2)$. By applying this inequality to each pair of square roots, we can show that $\sqrt{a}+\sqrt{b}+\sqrt{c}+\sqrt{d}\le \sqrt{2(a+b+c+d)}$. Then, using the fact that the square root function is concave, we can show that $\sqrt{2(a+b+c+d)} \le 10$, thus proving the original inequality.
Yes, the inequality can be generalized to any number of terms. In fact, it is a special case of the generalized Minkowski inequality, which states that for any positive numbers $x_1, x_2, ..., x_n$ and any positive real number p, we have $(x_1^p + x_2^p + ... + x_n^p)^{\frac{1}{p}} \le x_1 + x_2 + ... + x_n$. In the case of our inequality, we have p = 0.5 and n = 4.
This inequality has various applications in mathematics and physics. For example, it can be used to prove the convergence of certain infinite series, as well as to establish the existence of solutions to certain differential equations. It also has applications in geometry, where it can be used to prove the existence of certain geometric constructions.
Yes, there are many other similar inequalities involving square roots. Some examples include the Cauchy-Schwarz inequality, the AM-GM inequality, and the Jensen's inequality. These inequalities have various applications in different areas of mathematics and can be used to prove many other important results.