Răspuns:
[tex]3)a \div 3 + (123 \times 3 - 54 \times 4) = 357 - 124 \div 4 \\ a \div 3 + (369 - 216) = 357 - 31 \\ a \div 3 + 153 = 326 \\ a \div 3 = 326 - 153 \\ a \div 3 = 173 \\ a = 173 \times 3 \\ a = 519[/tex]
[tex]4)(1 + 2 + 3 + .. + 15) = \frac{15(1 + 15)}{2} = \frac{15 \times 16}{2} = \frac{240}{2} = 120 \\ (2 + 4 + 6 + ... + 20) = 2(1 + 2 + 3 + .. + 10) = 2 \times \frac{10(1 + 10)}{2} = 2 \times \frac{10 \times 11}{2} = 2 \times \frac{110}{2} = \frac{220}{2} = 110 \\ (1 + 3 + 5 + .. + 11) = \frac{((11 - 1) \div 2 + 1)(1 + 11)}{2} = \frac{(10 \div 2 + 1) \times 12}{2} = \frac{6 \times 12}{2} = \frac{72}{2} = 36 \\ {(1 + 3 + 5 + .... + 11)}^{2} = {36}^{2} \\ (1 + 2 + 3 + .. + 15) + (2 + 4 + 6 + .. + 20) + {(1 + 3 + 5 + .. + 11)}^{2} = 120 + 110 + {36}^{2} = 230 + 1296 = 1526[/tex]
