[tex]a=2 \sqrt{50} -3 \sqrt{128} +4 \sqrt{72} -5 \sqrt{162} +6 \sqrt{98} \\ a=2 \cdot 5 \sqrt{2} -3 \cdot 8 \sqrt{2} +4 \cdot 6 \sqrt{2} -5 \cdot 9 \sqrt{2} +6 \cdot 7 \sqrt{2} \\ a=10 \sqrt{2} -24 \sqrt{2} +24 \sqrt{2} -45 \sqrt{2} +42 \sqrt{2} \\ a=7 \sqrt{2} [/tex]