2) \(\sqrt{98}-\sqrt{72}+0,5\sqrt{8}\)

\(=7\sqrt{2}-6\sqrt{2}+\sqrt{2}\)

\(=\left(7-6+1\right)\sqrt{2}\)

\(=2\sqrt{2}\)

3) \(\sqrt{9a}-\sqrt{16a}+\sqrt{49a}\)

\(=3\sqrt{a}-4\sqrt{a}+7\sqrt{a}\)

\(=\left(3-4+7\right)\sqrt{a}\)

\(=6\sqrt{a}\)

4) \(\sqrt{16b}+2\sqrt{40b}-3\sqrt{90b}\)

\(=4\sqrt{b}+4\sqrt{10b}-9\sqrt{10b}\)

\(=4\sqrt{b}-5\sqrt{10b}\)

Gấp nha 

a) Ta có: \(5\sqrt{a}-3\sqrt{25a^3}+2\sqrt{36ab^2}-2\sqrt{9a}\)

\(=5\sqrt{a}-15a\sqrt{a}+12b\sqrt{a}-6\sqrt{a}\)

\(=-\sqrt{a}-15a\sqrt{a}+12\sqrt{a}b\)

b) Ta có: \(\sqrt{64ab^3}-3\sqrt{12a^3b^3}+2ab\sqrt{9ab}-5b\sqrt{81a^3b}\)

\(=8b\sqrt{a}-6ab\sqrt{3ab}+6ab\sqrt{ab}-45a^2b\sqrt{ab}\)

a)\(5\sqrt{a}-3\sqrt{25a^3}+2\sqrt{36ab^2}-2\sqrt{9a}=5\sqrt{a}-15\left|a\right|\sqrt{a}+12\left|b\right|\sqrt{a}-6\sqrt{a}=-\sqrt{a}-15a\sqrt{a}+12b\sqrt{a}\)

b)\(\sqrt{64ab^3}-3\sqrt{12a^3b^3}+2ab\sqrt{9ab}-5b\sqrt{81a^3b}\)

\(=8\left|b\right|\sqrt{ab}-6\left|ab\right|\sqrt{3ab}+6ab\sqrt{ab}-45b\left|a\right|\sqrt{ab}\)

\(=8b\sqrt{ab}-6ab\sqrt{3ab}+6ab\sqrt{ab}-45ab\sqrt{ab}\)

\(=8b\sqrt{ab}-6ab\sqrt{3ab}-39ab\sqrt{ab}\)

a) \(a-\sqrt{a}\)

b) \(-5ab\sqrt{ab}\)

a) Ta có:

\(5\sqrt{a}-4b\sqrt{25a^3}+5a\sqrt{16ab^2}-2\sqrt{9a}\)

\(=5\sqrt{a}-4b.5a\sqrt{a}+5a.4b\sqrt{a}-2.3\sqrt{a}\)

\(=5\sqrt{a}-20ab\sqrt{a}+20ab\sqrt{a}-6\sqrt{a}\) \(=-\sqrt{a}\)

b) Ta có:

\(5a\sqrt{64ab^3}-\sqrt{3}.\sqrt{12a^3b^3}+2ab\sqrt{9ab}\) \(-5b\sqrt{81a^3b}\)

\(=5a.8b\sqrt{ab}-\sqrt{3.12a^3b^3}+2ab.3\sqrt{ab}\) \(-5b.9a\sqrt{ab}\)

\(=40ab\sqrt{ab}-6ab\sqrt{ab}+6ab\sqrt{ab}-45ab\)\(\sqrt{ab}\)

\(=-5ab\sqrt{ab}\)

a)5\(\sqrt{a}\)\(-4b\sqrt{25a^3}+5a\sqrt{16ab^2}-2\sqrt{9a}\)

=5\(\sqrt{a}-20ab\sqrt{a}+20ab\sqrt{a}-6\sqrt{a}\)

= \(\sqrt{a}\left(5-20ab+20ab-6\right)\)

= -\(\sqrt{a}\)

\(a,x=16\Rightarrow A=\dfrac{\sqrt{16}+2}{\sqrt{16}-3}=\dfrac{4+2}{4-3}=6\)

\(b,B=\dfrac{\sqrt{x}+5}{\sqrt{x}+1}+\dfrac{\sqrt{x}-7}{1-x}\left(dk:x\ge0,x\ne1,x\ne9\right)\\ =\dfrac{\sqrt{x}+5}{\sqrt{x}+1}-\dfrac{\sqrt{x}-7}{\left(\sqrt{x}-1\right)\left(\sqrt{x}+1\right)}\\ =\dfrac{\left(\sqrt{x}+5\right)\left(\sqrt{x}-1\right)-\left(\sqrt{x}-7\right)}{x-1}\\ =\dfrac{x+4\sqrt{x}-5-\sqrt{x}+7}{x-1}\\ =\dfrac{x+3\sqrt{x}+2}{x-1}\\ =\dfrac{\left(\sqrt{x}+2\right)\left(\sqrt{x}+1\right)}{\left(\sqrt{x}-1\right)\left(\sqrt{x}+1\right)}\\ =\dfrac{\sqrt{x}+2}{\sqrt{x}-1}\left(dpcm\right)\)

\(c,\dfrac{4A}{A}\le\dfrac{x}{\sqrt{x}-3}\Leftrightarrow\dfrac{4\left(\sqrt{x}+2\right)}{\sqrt{x}-3}:\dfrac{\sqrt{x}+2}{\sqrt{x}-3}\le\dfrac{x}{\sqrt{x}-3}\)

\(\Leftrightarrow\dfrac{4\left(\sqrt{x}+2\right)}{\sqrt{x}-3}.\dfrac{\sqrt{x}-3}{\sqrt{x}+2}\le\dfrac{x}{\sqrt{x}-3}\)

\(\Leftrightarrow4-\dfrac{x}{\sqrt{x}-3}\le0\)

\(\Leftrightarrow\dfrac{4\sqrt{x}-12-x}{\sqrt{x}-3}\le0\)

\(\Leftrightarrow\) Pt vô nghiệm

Vậy không có giá trị x thỏa yêu cầu đề bài.

Do \(a\le\left|a\right|,b\le\left|b\right|\) nên ta chỉ cần chứng minh

\(\dfrac{\left|a\right|}{\sqrt{a^2+b^2}}+\dfrac{\left|b\right|}{\sqrt{9a^2+b^2}}+\dfrac{2\left|a\right|\left|b\right|}{\sqrt{a^2+b^2}.\sqrt{9a^2+b^2}}\le\dfrac{3}{2}\)

Đặt \(a^2=x,b^2=3y^2\)

\(P=2\sqrt{\dfrac{x}{x+3y}}+2\sqrt{\dfrac{y}{y+3x}}+4\sqrt{\dfrac{xy}{\left(x+3y\right)\left(y+3x\right)}}\le3\)

Sử dụng BĐT AM-GM, ta có

\(2\sqrt{\dfrac{x}{x+3y}}\le\dfrac{x}{x+y}+\dfrac{x+y}{3x+y},2\sqrt{\dfrac{y}{y+3x}}\le\dfrac{y}{x+y}+\dfrac{x+y}{y+3x}\)\(4\sqrt{\dfrac{xy}{\left(x+3y\right)\left(y+3x\right)}}\le\dfrac{8xy}{\left(x+3y\right)\left(y+3x\right)}+\dfrac{1}{2}\)

Cộng ba bất đẳng thức trên vế theo vế

\(P\le\dfrac{3}{2}+\dfrac{x+y}{x+3y}+\dfrac{x+y}{y+3x}+\dfrac{8xy}{\left(x+3y\right)\left(y+3x\right)}\)

Và do đó chứng minh sẽ hoàn tất nếu ta chỉ ra được rằng:

\(\dfrac{x+y}{x+3y}+\dfrac{x+y}{y+3x}+\dfrac{8xy}{\left(x+3y\right)\left(y+3x\right)}\le\dfrac{3}{2}\)

Ta có: \(\dfrac{3}{2}-\dfrac{x+y}{x+3y}-\dfrac{x+y}{y+3x}-\dfrac{8xy}{\left(x+3y\right)\left(y+3x\right)}=\dfrac{3}{2}-\dfrac{4\left(x+y\right)^2+8xy}{\left(x+3y\right)\left(y+3x\right)}=\dfrac{\left(x-y\right)^2}{2\left(x+3y\right)\left(y+3x\right)}\ge0\)Bài toán được chứng minh xong. Đẳng thức xảy ra khi \(b=\sqrt{3}a>0\)

ĐS: a)

b)

c)

d)

a) \(\sqrt{75}+\sqrt{48}-\sqrt{300}\) = \(5\sqrt{3}+4\sqrt{3}-10\sqrt{3}\) = \(-\sqrt{3}\)

b) \(\sqrt{98}-\sqrt{72}+0,5\sqrt{8}\) = \(7\sqrt{2}-6\sqrt{2}+\sqrt{2}\) = \(2\sqrt{2}\)

c) \(\sqrt{9a}-\sqrt{16a}+\sqrt{49a}\) = \(3\sqrt{a}-4\sqrt{a}+7\sqrt{a}\) = \(6\sqrt{a}\)

d) \(\sqrt{16b}+2\sqrt{40b}-3\sqrt{90b}\) = \(4\sqrt{b}+4\sqrt{10b}-9\sqrt{10b}\)

= \(4\sqrt{b}-5\sqrt{10b}\)

\(\sqrt{16b}+2\sqrt{40b}-3\sqrt{90b}=4\sqrt{b}+2.2\sqrt{10b}-3.3\sqrt{10b}=4\sqrt{b}+4\sqrt{10b}-9\sqrt{10b}=4\sqrt{b}-5\sqrt{10b}\)

\(A=\dfrac{\sqrt{x}\left(x\sqrt{x}-1\right)}{x+\sqrt{x}+1}-\dfrac{\sqrt{x}\left(2\sqrt{x}+1\right)}{\sqrt{x}}+\dfrac{2\left(\sqrt{x}-1\right)\left(\sqrt{x}+1\right)}{\sqrt{x}-1}\\ A=\dfrac{\sqrt{x}\left(\sqrt{x}-1\right)\left(x+\sqrt{x}+1\right)}{x+\sqrt{x}+1}-\left(2\sqrt{x}+1\right)+2\left(\sqrt{x}+1\right)\\ A=x-\sqrt{x}-2\sqrt{x}-1+2\sqrt{x}+2=x-\sqrt{x}+1\)

\(B=\dfrac{7a-7b+8a+8b-16b}{\left(a+b\right)\left(a-b\right)}=\dfrac{15a-15b}{\left(a-b\right)\left(a+b\right)}\\ B=\dfrac{15\left(a-b\right)}{\left(a-b\right)\left(a+b\right)}=\dfrac{15}{a+b}\)