Sum of the first 13 term an arithmetic progression if the 5th trem is 1 and the 8th is -17
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S13 = = -143
Step-by-step explanation:
t5 = 1
a + 4d = 1..........(1)
t8 = -17
a + 7d = -17.......(2)
sub eq (1) and (2)
a + 4d = 1 - a + 7d = -17 = -3d = 18
d = -18/3
d = -6
put value of d in eq..(1)
a + 4(-6) = 1
a - 24 = 1
a = 1 + 24
a = 25
S13 = n/2[2a + ( n + 1)d]
= 13/2[2 × 25 + (13 - 1)-6
= 13/2[2 × 25 + 12 × (-6)
= 13/2[50 × (-72)
= 13/2[-22]
= 13/2 × (-22)
= 13 × (-11)
= -143
Thank ❤️
silentlover45.❤️
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