Mà h cóa ai bik làm ko giúp mk dới :)) 

\(D=\dfrac{11}{4}\cdot\left(-0,4\right)-\dfrac{8}{5}\cdot2,75+\left(-1,2\right):\dfrac{4}{11}\)

\(D=\dfrac{11}{4}\cdot\left(\dfrac{-2}{5}\right)-\dfrac{8}{5}\cdot\dfrac{11}{4}+\left(\dfrac{-6}{5}\right)\cdot\dfrac{11}{4}\)

\(D=\dfrac{11}{4}\cdot\) \(\left(\dfrac{-2}{5}-\dfrac{8}{5}+\dfrac{-6}{5}\right)\)

\(D=\dfrac{11}{4}\cdot\left(-\dfrac{16}{5}\right)\)

\(D=-\dfrac{44}{5}\)

a: Không vì \(6\cdot\left(-3\right)< >4\cdot4.5\)

2x^2-x-2020=0

=>x=(1+căn 16161)/4 hoặc x=(1-căn 16161)/4

Gọi A(1+căn 16161/4;0); B(1-căn 16161/4;0); N(0;b)

\(AB=\dfrac{\sqrt{2\cdot\sqrt{16161}}}{2};AN=\sqrt{\left(0-\dfrac{1+\sqrt{16161}}{4}\right)^2+\left(b-0\right)^2}\)

\(BN=\sqrt{\left(0-\dfrac{1-\sqrt{16161}}{4}\right)^2+\left(b-0\right)^2}\)

ΔABN vuông tại N

=>NA^2+NB^2=AB^2

=>\(\left(\dfrac{1+\sqrt{16161}}{4}\right)^2+b^2+\left(\dfrac{1-\sqrt{16161}}{4}\right)^2+b^2=\left(\dfrac{1+\sqrt{16161}}{4}-\dfrac{1-\sqrt{16161}}{4}\right)^2\)

=>b^2=-2(1-16161)/16*2=1010

=>b=căn 1010

Công của người đó là

\(A=P.h=10m.h=10.60.4=2400\left(J\right)\) 

Công suất trong 2p (120s) là

\(P=\dfrac{A}{t}=\dfrac{2400}{20}=120\left(W\right)\)

\(8\dfrac{1}{2}=\dfrac{17}{2};9\dfrac{3}{4}=\dfrac{39}{4};12\dfrac{2}{3}=\dfrac{38}{3};7\dfrac{5}{14}=\dfrac{103}{14}\)

\(8\dfrac{1}{2}=\dfrac{17}{2};9\dfrac{3}{4}=\dfrac{39}{4};12\dfrac{2}{3}=\dfrac{38}{3}=7\dfrac{5}{14}=\dfrac{103}{14}\)