Mình tl k mình nha

A= a .1/2+a .1/3-a .1/4 với a=-4/5

A=a.(1/2+1/3-1/4)

A=-4/5.(6/12+4/12-3/12)

A=-4/5 . 7/12

A=\(\frac{-7}{15}\)

1: A(-1;1); B(0;-1); C(1;-3)

\(\overrightarrow{AB}=\left(0+1;-1-1\right)=\left(1;-2\right)\)

\(\overrightarrow{AC}=\left(1+1;-3-1\right)=\left(2;-4\right)\)

Vì \(\frac12=\frac{-2}{-4}\left(=\frac12\right)\)

nên A,B,C thẳng hàng

2: A(2;0); B(5;1); C(-1;-1)

\(\overrightarrow{AB}=\left(5-2;1-0\right)=\left(3;1\right);\overrightarrow{AC}=\left(-1-2;-1-0\right)=\left(-3;-1\right)\)

Vì \(\frac{3}{-3}=\frac{1}{-1}\left(=-1\right)\)

nên A,B,C thẳng hàng

3: A(4;3); B(2;0); C(0;-3)

\(\overrightarrow{AB}=\left(2-4;0-3\right)=\left(-2;-3\right);\overrightarrow{AC}=\left(0-4;-3-3\right)=\left(-4;-6\right)\)

Vì \(\frac{-2}{-4}=\frac{-3}{-6}\left(=\frac12\right)\)

nên A,B,C thẳng hàng

4: A(-1;2); B(2;3); C(4;-1)

\(\overrightarrow{AB}=\left(2+1;3-2\right)=\left(3;1\right);\overrightarrow{AC}=\left(4+1;-1-2\right)=\left(5;-3\right)\)
Vì 3/5<>1/-3

nên A,B,C không thẳng hàng

a) = =

b) = = = . ( Với điều kiện b # 1)

c) \(\dfrac{a^{\dfrac{1}{3}}b^{-\dfrac{1}{3}-}a^{-\dfrac{1}{3}}b^{\dfrac{1}{3}}}{\sqrt[3]{a^2}-\sqrt[3]{b^2}}\)= = = ( với điều kiện a#b).

d) \(\dfrac{a^{\dfrac{1}{3}}\sqrt{b}+b^{\dfrac{1}{3}}\sqrt{a}}{\sqrt[6]{a}+\sqrt[6]{b}}\) = = = =

a)Ta có :

\(A=\dfrac{1}{4}+\dfrac{1}{4^2}+\dfrac{1}{4^3}+............+\dfrac{1}{4^{100}}\)

\(4A=1+\dfrac{1}{4}+\dfrac{1}{4^2}+\dfrac{1}{4^3}+..........+\dfrac{1}{4^{99}}\)

\(4A-A=\left(1+\dfrac{1}{4}+.......+\dfrac{1}{4^{99}}\right)-\left(\dfrac{1}{4}+\dfrac{1}{4^2}+.....+\dfrac{1}{4^{100}}\right)\)

\(3A=1-\dfrac{1}{4^{100}}\)

\(\Rightarrow A=\dfrac{1-\dfrac{1}{4^{100}}}{3}\)

~ Chúc bn học tốt ~

Chọn A

A nhé