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D( a )
a^2-81 = 0
a^2-(7*a)-18 = 0
a^2-81 = 0
a^2-81 = 0
1*a^2 = 81 // : 1
a^2 = 81
a^2 = 81 // ^ 1/2
abs(a) = 9
a = 9 or a = -9
a^2-(7*a)-18 = 0
a^2-(7*a)-18 = 0
a^2-7*a-18 = 0
a^2-7*a-18 = 0
DELTA = (-7)^2-(-18*1*4)
DELTA = 121
DELTA > 0
a = (121^(1/2)+7)/(1*2) or a = (7-121^(1/2))/(1*2)
a = 9 or a = -2
a in (-oo:-9) U (-9:-2) U (-2:9) U (9:+oo)
(((a^2-a-6)/(a^2-81))*(4*a+36))/(a^2-(7*a)-18) = 0
(((a^2-a-6)/(a^2-81))*(4*a+36))/(a^2-7*a-18) = 0
((a^2-a-6)*(4*a+36))/((a^2-81)*(a^2-7*a-18)) = 0
a^2-a-6 = 0
a^2-a-6 = 0
DELTA = (-1)^2-(-6*1*4)
DELTA = 25
DELTA > 0
a = (25^(1/2)+1)/(1*2) or a = (1-25^(1/2))/(1*2)
a = 3 or a = -2
(a+2)*(a-3) = 0
a^2-7*a-18 = 0
a^2-7*a-18 = 0
DELTA = (-7)^2-(-18*1*4)
DELTA = 121
DELTA > 0
a = (121^(1/2)+7)/(1*2) or a = (7-121^(1/2))/(1*2)
a = 9 or a = -2
(a+2)*(a-9) = 0
((a+2)*(a-3)*(4*a+36))/((a^2-81)*(a+2)*(a-9)) = 0
( 4*a+36 )
4*a+36 = 0 // - 36
4*a = -36 // : 4
a = -36/4
a = -9
( a+2 )
a+2 = 0 // - 2
a = -2
( a-3 )
a-3 = 0 // + 3
a = 3
a in { -9}
a in { -2}
a = 3
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