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D( a )
a/2+80 = 0
a/2+80 = 0
a/2+80 = 0
1/2*a+80 = 0 // - 80
1/2*a = -80 // : 1/2
a = -80/1/2
a = -160
a in (-oo:-160) U (-160:+oo)
0.2 = 80-(a/(a/2+80)) // - 80-(a/(a/2+80))
a/(a/2+80)-80+0.2 = 0
a/(1/2*a+80)-80+0.2 = 0
a/(1/2*a+80)+(-80*(1/2*a+80))/(1/2*a+80)+(0.2*(1/2*a+80))/(1/2*a+80) = 0
0.2*(1/2*a+80)-80*(1/2*a+80)+a = 0
0.1*a-39*a-6400+16 = 0
-38.9*a-6384 = 0
(-38.9*a-6384)/(1/2*a+80) = 0
(-38.9*a-6384)/(1/2*a+80) = 0 // * 1/2*a+80
-38.9*a-6384 = 0
-38.9*a-6384 = 0 // + 6384
-38.9*a = 6384 // : -38.9
a = 6384/(-38.9)
a = -164.11311054
a = -164.11311054
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