حل السؤال 18، 4MA1/1HR يونيو 2020: مجموع المتتابعة الحسابية

1. 1a + 5d = 39
2. 2a + 18d = 7.8
3. 3Subtract the first equation from the second: (a + 18d) − (a + 5d) = 7.8 − 39
4. 413d = −31.2
5. 5d = −31.2 / 13 = −2.4
6. 6Substitute d = −2.4 into a + 5d = 39: a + 5(−2.4) = 39
7. 7a − 12 = 39
8. 8a = 51
9. 9Use the sum formula: S_n = n/2 [2a + (n-1)d]
10. 10S_25 = 25/2 [2(51) + (25-1)(−2.4)]
11. 11S_25 = 12.5 [102 + 24(−2.4)]
12. 12S_25 = 12.5 [102 − 57.6]
13. 13S_25 = 12.5 [44.4]
14. 14S_25 = 555

Answer555