A = 4x + 7 - \(\sqrt{16x^2+56x+49}\)
A = 4x + 7 - \(\sqrt{\left(4x+7\right)^2}\)
A = 4x + 7 - \(|4x+7|\)
A = \(\left[{}\begin{matrix}4x+7-\left[-\left(4x+7\right)\right]\\4x+7-\left(4x+7\right)\end{matrix}\right.\)
A = \(\left[{}\begin{matrix}4x+7+4x+7\\4x+7-4x-7\end{matrix}\right.\)
A = \(\left[{}\begin{matrix}8x+14\\0\end{matrix}\right.\)
Thay x = 3 vào 8x + 14, ta được:
8 . 3 + 14
= 24 + 14
= 38
a: Ta có: \(\sqrt{4x^2-9}=2\sqrt{2x+3}\)
\(\Leftrightarrow4x^2-9=8x+12\)
\(\Leftrightarrow4x^2-8x-21=0\)
\(\text{Δ}=\left(-8\right)^2-4\cdot4\cdot\left(-21\right)=400\)
Vì Δ>0 nên phương trình có hai nghiệm phân biệt là:
\(\left\{{}\begin{matrix}x_1=\dfrac{8-20}{8}=\dfrac{-12}{8}=-\dfrac{3}{2}\left(nhận\right)\\x_2=\dfrac{8+20}{8}=\dfrac{28}{8}=\dfrac{7}{2}\left(nhận\right)\end{matrix}\right.\)
