
1) x=9, because we can use the Pythagorean theorem, x^2 + 144 = 225, x^2 = 81
2)
, because 6^2 +6^2 = 72, 
3)
, because 
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Keep your current environment well-ventilated.
Make sure to wear a mask if needed.
Open windows and/or doors.
Eat while you work. Having a healthy snack while you complete your homework, such as fruits or vegetables, can actually sharpen your focus and improve your memory.
Reward yourself for each task accomplished.
Think of the free time you will have to enjoy once your homework is complete.