B = 45a + 79b + 30a – 54b với 3a + b = 2021.
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![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(1) \sqrt{9a^2.b^2}\)=3ab
\(2) \sqrt{3a}.\sqrt{27a}=\sqrt{3a}.3\sqrt{3a}=9a\)
\(3) \sqrt{3a^5}.12a=12\sqrt{3a^7}\)
\(4) \sqrt{5a}.\sqrt{45a}-3a=15a-3a=12a\)
\(5) \sqrt{3+\sqrt{a}}.\sqrt{3-\sqrt{a}}=\sqrt{(3+\sqrt{a}).(3-\sqrt{a})} =\sqrt{9-a} \)
\(6) \sqrt{3+\sqrt{5}}.\sqrt{3\sqrt{5}} =\sqrt{\sqrt{3\sqrt{5}}.(3+\sqrt{5})} =\sqrt{9+\sqrt{15}}\)
1) \(\sqrt{9a^2b^2}=3ab\)
2) \(\sqrt{3a}\cdot\sqrt{27a}=9a\)
4) \(\sqrt{5a}\cdot\sqrt{45a}-3a=15a-3a=12a\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a: \(\dfrac{\sqrt{2}+\sqrt{3}+\sqrt{6}+\sqrt{8}+\sqrt{16}}{\sqrt{2}+\sqrt{3}+\sqrt{4}}\)
\(=\dfrac{\sqrt{2}+\sqrt{3}+\sqrt{4}+\sqrt{4}+\sqrt{6}+\sqrt{8}}{\sqrt{2}+\sqrt{3}+\sqrt{4}}\)
\(=1+\sqrt{2}\)
b: \(\sqrt{\dfrac{2a}{3}}\cdot\sqrt{\dfrac{3a}{8}}=\sqrt{\dfrac{6a^2}{24}}=\sqrt{\dfrac{a^2}{4}}=\dfrac{a}{2}\)
c: \(\sqrt{5a\cdot45a}-3a=-15a-3a=-18a\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Do a ≥ 0 nên bài toán luôn xác định. Ta có:
(Vì a ≥ 0 nên |a| = a)
![](https://rs.olm.vn/images/avt/0.png?1311)
a) Ta có: \(\sqrt{27\cdot48\left(1-a^2\right)}\)
\(=\sqrt{3^4\cdot4^2\cdot\left(1-a^2\right)}\)
\(=36\sqrt{1-a^2}\)
c) Ta có: \(\sqrt{5a}\cdot\sqrt{45a}-3a\)
\(=15a-3a=12a\)
b) Ta có: \(B=\dfrac{1}{a-b}\cdot\sqrt{a^4\cdot\left(a-b\right)^2}\)
\(=\dfrac{1}{a-b}\cdot a^2\cdot\left(a-b\right)\)
\(=a^2\)
d) Ta có: \(D=\left(3-a\right)^2-\sqrt{0.2}\cdot\sqrt{180a^2}\)
\(=a^2-6a+9-\sqrt{36a^2}\)
\(=a^2-6a+9-\left|6a\right|\)
\(=\left[{}\begin{matrix}a^2-6a+9-6a\left(a\ge0\right)\\a^2-6a+9+6a\left(a< 0\right)\end{matrix}\right.\)
\(=\left[{}\begin{matrix}a^2-12a+9\\a^2+9\end{matrix}\right.\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a,Ta có: \(\sqrt{3a}.\sqrt{27a}-5a=\sqrt{3.3^3.a^2}-5a=\left|3^2a\right|-5a=9a-5a=4a\)
b,\(\left(2-a\right)^2-\sqrt{0,3.30a^2}=4-4a+a^2-\left|3a\right|\)
\(\Rightarrow\left[{}\begin{matrix}=a^2-7a+4\\=a^2-a+4\end{matrix}\right.\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Giup minh voi
`B=45a+79b+30a-54b`
`=75a+25b`
`=25(3a+b)`
`=25.2021`
`=50525`