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145+(a+2×a+3×a+.....+8×a)×12=2305

Răspuns :

145+(a+2a+3a+...+8a)*12=2305 <=> (a+2a+3a+...+8a)*12=2305-145 <=> (1+2+3+...+8)*12a=2160 <=> (8*9/2)*12a=2160 <=> 36*12a=2160 <=> 432a=2160 <=> a=5.