c: =5a(a-b)+7(a-b)

=(a-b)(5a+7)

x₁=1

x₂=\(\dfrac{-2}{7}\)

\(21x^3-15x^2-6x=0\\ \Leftrightarrow x\left(21x^2-15x-6\right)=0\\ \Leftrightarrow x\left[\left(21x^2-21x\right)+\left(6x-6\right)\right]=0\\ \Leftrightarrow x\left[21x\left(x-1\right)+6\left(x-1\right)\right]=0\\ \Leftrightarrow x\left(x-1\right)\left(21x+6\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=0\\x=1\\x=\dfrac{-2}{7}\end{matrix}\right.\)

\(21x^3-15x^2-6x=0\)

\(\Leftrightarrow21x^3-21x^2+6x^2-6x=0\)

\(\Leftrightarrow21x^2\left(x-1\right)+6x\left(x-1\right)=0\)

\(\Leftrightarrow\left(21x^2+6x\right)\left(x-1\right)=0\)

\(\Leftrightarrow3x\left(7x+2\right)\left(x-1\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}3x=0\\7x+2=0\\x-1=0\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=0\\x=-\dfrac{2}{7}\\x=1\end{matrix}\right.\)

(30x-1)+(29x-2)+...+(21x-10)=710

<=>[(30-10):1+1]x-(1+2+...+10)=710

<=>21x-[(10+1).10:2]=710

<=>21x-55=710

=>21x=710-55

=>21x=655

=>x=655/21

Chúc bạn học giỏi nha!!!

(30x-1)+(29x-2)+...+(21x-10)=710

<=>[(30-10):1+1]x-(1+2+...+10)=710

<=>21x-[(10+1).10:2]=710

<=>21x-55=710

=>21x=710-55

=>21x=655

=>x=655/21

Chúc bạn học giỏi nha!!!

\(x=\sqrt{\sqrt{3}-\sqrt{5-\sqrt{13+4\sqrt{3}}}}\)

\(=\sqrt{\sqrt{3}-\sqrt{4-2\sqrt{3}}}\)

=1

Thay x=1 vào P, ta được:
\(P=\left(21\cdot1^2+6\cdot1\right)^{2017}=27^{2017}\)

a: \(\Leftrightarrow4x^3+16x^2+28x-x^2-4x-7+10+a⋮x^2+4x+7\)

hay a=-10