解答

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17.
a^2 -6a +9 +b^2 -10b +25 =0
(a-3)^2 +(b-5)^2 =0
=> a = 3 、b = 5
=> 2a+3b = 21

18.
a^2 -4a +4 +b^2 -10b +25 =0
(a-2)^2 +(b-5)^2 =0
=> a = 2 、b = 5
=> a+b = 7

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