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mark and his friend are thirteen years old.and they are very sporty. they often go to the gym - at least three timesa week. mark does exercises to make his arms strong.he can t walk because of a car accident, so he goes everywhere in wheelchair. but it doesn t stop him doing the things that boys our age want to do.

s = pi*r^2

r=r/2  

подставим

s=pi*(r/2)^2

s=(pi*r^2)/4

небольшая муз. пьеса, чаще всего для фортепиано, но также и для др. инструментов, для камерных ансамблей, оркестра

на органе и клавесине