có ai giúp mình câu này với mai phải nộp rồi

B
D
C

a=-1/2.8

a=-4

7: \(\dfrac{a}{2\sqrt{a}-1}=\dfrac{a\left(2\sqrt{a}+1\right)}{4a-1}\)

9: \(\dfrac{1}{\sqrt{a}+\sqrt{a+1}}=\dfrac{\sqrt{a+1}-\sqrt{a}}{a+1-a}=\sqrt{a+1}-\sqrt{a}\)

mn giúp em vs ạ em cần gấp cảm ơn ạ 

d. \(\dfrac{\pi}{2}< a;b< \pi\Rightarrow sina>0;sinb>0\)

\(sina=\sqrt{1-cos^2a}=\dfrac{4}{5}\Rightarrow tana=\dfrac{sina}{cosa}=-\dfrac{4}{3}\)

\(sinb=\sqrt{1-cos^2b}=\dfrac{5}{13}\Rightarrow tanb=-\dfrac{5}{12}\)

Vậy:

\(sin\left(a-b\right)=sina.cosb-cosa.sinb=\dfrac{4}{5}.\left(-\dfrac{12}{13}\right)-\left(-\dfrac{3}{5}\right)\left(\dfrac{5}{13}\right)=...\)

\(cos\left(a-b\right)=cosa.cosb-sina.sinb=...\) (bạn tự thay số bấm máy)

\(tan\left(a+b\right)=\dfrac{tana+tanb}{1-tana.tanb}=...\)

\(cot\left(a+b\right)=\dfrac{1}{tan\left(a+b\right)}=\dfrac{1-tana.tanb}{tana+tanb}=...\)

e.

\(0< y< \dfrac{\pi}{2}\Rightarrow cosy>0\Rightarrow cosy=\sqrt{1-sin^2y}=\dfrac{4}{5}\)

\(\Rightarrow tany=\dfrac{siny}{cosy}=\dfrac{3}{4}\)

Vậy: \(tan\left(x+y\right)=\dfrac{tanx+tany}{1-tanx.tany}=...\)

\(cot\left(x-y\right)=\dfrac{1}{tan\left(x-y\right)}=\dfrac{1+tanx.tany}{tanx-tany}=...\)