Câu 14/
ĐK: $x\ge 1$
$\sqrt{4x-4}+\dfrac{2}{3}\sqrt{36x-36}-2\sqrt{9x-9}\\=\sqrt{4(x-1)}+\dfrac{2}{3}.\sqrt{36(x-1)}-2\sqrt{9(x-1)}\\=2\sqrt{x-1}+4\sqrt{x-1}-6\sqrt{x-1}\\=0\\→A$
Câu 15/ Diện tích quạt tròn là:
$\dfrac{\pi R^2 n}{360^\circ}=\dfrac{7\pi R^2}{24}\\→24\pi R^2 n=2520\pi R^2\\↔n=\dfrac{2520\pi R^2}{24\pi R^2}\\↔n=105^\circ\\→D$