1
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abc-ab-c=207
100a+10b+c-10a-b-c=207
90a+9b=207
10a+b=23
b=3
a=2
c={0,1,2,3,4,5,6,7,8,9}
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numerele de forma abc sunt:230,231,232,233,234,235,236,237,238,239
2.
(a+2b)(1/a+1/2b)=4
(a+2b)(2b+a)/2ab=4
(a+2b)²=8ab
a²+4ab+4b²=8ab
a²-4ab+4b²=0
a-2b=0
a=2b
2a+3b=√98
2a+3b=7√2
2×2b+3b=7√2
7b=7√2
b=√2
a=2√2