[tex]s1 = 10 + 11 + 12 + ... + 50 \\ nr \: de \: termeni = (50 - 10) \div 1 + 1 = 41 \\ s1 = \frac{(50 + 10) \times 41}{2} = 30 \times 41 \\ s1 = 1230 \\ s2 = 1 + 3 + 5 + 7 + ... + 99 \\ nr \: de \: termeni = (99 - 1) \div 2 + 1 = 45 \\ s2 = \frac{(99 + 1) \times 45}{2} = 50 \times 45 = 2250 \\ s3 = 2 + 4 + 6 + ... + 100 = \\ = 2(1 + 2 + 3 + ... + 50) = \\ = 2( \frac{50 \times 51}{2} ) = \\ = 50 \times 51 = \\ = 2550[/tex]
sper ca se vede
