Câu 1:
\(a,=\sqrt{49}\cdot\sqrt{81}=7\cdot9=63\\ b,=\sqrt{1,21}\cdot\sqrt{25}=1,1\cdot25=27,5\)
Câu 2:
\(a,=6\sqrt{2}-15\sqrt{2}+28\sqrt{2}=19\sqrt{2}\\ b,=\left(2\sqrt{3}-2\sqrt{5}+\sqrt{3}\right)\sqrt{3}+2\sqrt{15}=6-2\sqrt{15}+3+2\sqrt{15}=9\)
Câu 3:
\(a,\Leftrightarrow x=14^2=196\\ b,\Leftrightarrow\sqrt{x}=9\Leftrightarrow x=81\)
Câu 4:
\(a,BC=BH+HC=10\left(cm\right)\\ AH=\sqrt{BH\cdot HC}=4,8\left(cm\right)\\ AC=\sqrt{BC\cdot CH}=8\left(cm\right)\\ AB=\sqrt{BC\cdot BH}=6\left(cm\right)\\ b,\sin B=\dfrac{AC}{BC}=\dfrac{4}{5}\approx\sin53^0\\ \Rightarrow\widehat{B}\approx53^0\Rightarrow\widehat{C}=90^0-\widehat{B}\approx37^0\\ c,\dfrac{AD}{DC}=\dfrac{AB}{BC}=\dfrac{3}{5}\Rightarrow AD=\dfrac{3}{5}DC\\ AD+DC=AC\\ \Rightarrow\dfrac{8}{5}DC=8\Rightarrow DC=5\\ \Rightarrow AD=3\\ \Rightarrow BD=\sqrt{AD^2+AB^2}=3\sqrt{5}\approx6,708\left(cm\right)\)
