Bài 5 :
\(a,M=\sqrt{9x^4+6x^2+1}-\sqrt{9x^4}\\ =\sqrt{\left(3x^2\right)^2+2.3.x^2+1}-\sqrt{\left(3x^2\right)^2}\\ =\sqrt{\left(3x^2+1\right)^2}-\left|3x^2\right|\\ =\left|3x^2+1\right|-3x^2\\ =1\)
\(b,N=\sqrt{8-2\sqrt{7}}-\sqrt{8+2\sqrt{7}}\\ =\sqrt{\left(\sqrt{7}-1\right)^2}-\sqrt{\left(\sqrt{7}+1\right)^2}\\ =\left|\sqrt{7}-1\right|-\left|\sqrt{7}+1\right|\\ =\sqrt{7}-1-\sqrt{7}-1\\ =-2\)
Bài 6 :
\(a,\sqrt{4m^2}-7m\left(dk:m< 0\right)\\ =\sqrt{\left(2m\right)^2}-7m\\ =\left|2m\right|-7m\\ =-2m-7m\\ =-9m\\ b,\sqrt{25m^4}-4\sqrt{9m^2}\left(dk:m\ge0\right)\\ =\sqrt{\left(5m^2\right)^2}-4\sqrt{\left(3m\right)^2}\\ =\left|5m^2\right|-4.\left|3m\right|\\ =5m^2-12m\)
Bài 5:
a) \(M=\sqrt{9x^4+6x^2+1}-\sqrt{9x^4}\)
\(M=\sqrt{\left(3x^2+1\right)^2}-\sqrt{\left(3x^2\right)^2}\)
\(M=3x^2+1-3x^2\)
\(M=1\)
b) \(N=\sqrt{8-2\sqrt{7}}-\sqrt{8+2\sqrt{7}}\)
\(N=\sqrt{\left(\sqrt{7}\right)^2-2\cdot\sqrt{7}\cdot1+1^2}-\sqrt{\left(\sqrt{7}\right)^2+2\sqrt{7}\cdot1+1^2}\)
\(N=\sqrt{\left(\sqrt{7}-1\right)^2}-\sqrt{\left(\sqrt{7}+1\right)^2}\)
\(N=\sqrt{7}-1-\sqrt{7}-1\)
\(N=-2\)
giúp mình với mình đang gấp