a, 4.(-1/2 ) 2 + 1/2
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DK
5 tháng 6 2021
Áp dụng BĐT phụ \(a^2+b^2\ge\dfrac{1}{2}\left(a+b\right)^2\Leftrightarrow\left(a-b\right)^2\ge0\)
\(A\ge\dfrac{1}{2}\left(x+y+\dfrac{1}{x}+\dfrac{1}{y}\right)^2\ge\dfrac{1}{2}\left(x+y+\dfrac{4}{x+y}\right)^2=\dfrac{1}{2}\left(1+\dfrac{4}{1}\right)^2=\dfrac{25}{2}\)
Dấu "=" \(x=y=\dfrac{1}{2}\)
3 tháng 8 2023
a,a+1/4=2 3/4-1 1/2
a+1/2=5/4
a=5/4-1/2
a=3/4
b,a-7/4=13/4-7/9
a-7/4=89/36
a= 89/36+7/4
a=152/36
c,3/2-a=17/6-1/6
3/2-a=8/3
a= 3/2-8/3
a= -7/6
KV
4 tháng 11 2023
Bài 1
a) 3 2/5 - 1/2
= 17/5 - 1/2
= 34/10 - 5/10
= 29/10
b) 4/5 + 1/5 × 3/4
= 4/5 + 3/20
= 16/20 + 3/20
= 19/20
c) 3 1/2 × 1 1/7
= 7/2 × 8/7
= 4
d) 4 1/6 : 2 1/3
= 25/6 : 7/3
= 25/14
KV
4 tháng 11 2023
Bài 2
a) 3 × 1/2 + 1/4 × 1/3
= 3/2 + 1/12
= 18/12 + 1/12
= 19/12
b) 1 4/5 - 2/3 : 2 1/3
= 9/5 - 2/3 : 7/3
= 9/5 - 2/7
= 63/35 - 10/35
= 53/35
HN
19 tháng 1 2017
1/ \(3\left(2^2+1\right)\left(2^4+1\right)...\left(2^{128}+1\right)\)
\(=\left(2^2-1\right)\left(2^2+1\right)\left(2^4+1\right)...\left(2^{128}+1\right)\)
\(=\left(2^4-1\right)\left(2^4+1\right)...\left(2^{128}+1\right)\)
..................................................................
\(=\left(2^{128}-1\right)\left(2^{128}+1\right)=2^{256}-1\)
HN
19 tháng 1 2017
2/ Ta có: \(a+b+c=0\Leftrightarrow a+b=-c\)
\(\Leftrightarrow a^2+2ab+b^2=c^2\Leftrightarrow a^2+b^2-c^2=-2ab\)
\(\Leftrightarrow a^4+b^4+c^4+2a^2b^2-2b^2c^2-2c^2a^2=4a^2b^2\)
\(\Leftrightarrow a^4+b^4+c^4=2a^2b^2+2b^2c^2+2c^2a^2\)
Ta lại có: \(a^2+b^2+c^2=10\)
\(\Leftrightarrow a^4+b^4+c^4+2a^2b^2+2b^2c^2+2c^2a^2=100\)
\(\Leftrightarrow2\left(a^4+b^4+c^4\right)=100\Leftrightarrow a^4+b^4+c^4=50\)
\(\Leftrightarrow\frac{1}{a^4+b^4+c^4}=\frac{1}{50}\)
TT
0
TT
0
TD
8
26 tháng 6 2023
15: A= 1/3-3/4+3/5+1/2007-1/36+1/15-2/9
Sửa đề:
A=-3/4-2/9-1/36+1/3+3/5+1/15+1/2007
=-27/36-8/36-1/36+5/15+9/15+1/15+1/2007
=-1+1+1/2007=1/2007
16:
\(A=\dfrac{1}{3}+\dfrac{3}{5}+\dfrac{1}{15}-\dfrac{3}{4}-\dfrac{2}{9}-\dfrac{1}{36}+\dfrac{1}{64}\)
\(=\dfrac{5+9+1}{15}+\dfrac{-27-8-1}{36}+\dfrac{1}{64}\)
=1/64
17:
=1/2-1/2+2/3-2/3+3/4-3/4+4/5-4/5+5/6-5/6-6/7
=-6/7
\(4\cdot\left(-\dfrac{1}{2}\right)^2+\dfrac{1}{2}\)
\(=4\cdot\dfrac{1}{4}+\dfrac{1}{2}\)
\(=1+\dfrac{1}{2}=\dfrac{3}{2}\)