find the sum of first 17th terms of AP whose 4th and 9th term is -15 and -30 respectively
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As we know
an=a+(n-1)d
a4= a+(3)d
-15= a+ 3d........eq (1)
Similarly,
-30=a+ 8d........eq(2)
on subtracting eq (2) from (1), we get
-30+15=a-a+8d-5d
-15=3d
d=-5
put the value of d in eq (1)
-15=a+3(-5)
a=0
Sn= 2a + (n-1)d
S17= 2(0) + (17-1)×-5
=0+16×-5
= -80.........ans.
an=a+(n-1)d
a4= a+(3)d
-15= a+ 3d........eq (1)
Similarly,
-30=a+ 8d........eq(2)
on subtracting eq (2) from (1), we get
-30+15=a-a+8d-5d
-15=3d
d=-5
put the value of d in eq (1)
-15=a+3(-5)
a=0
Sn= 2a + (n-1)d
S17= 2(0) + (17-1)×-5
=0+16×-5
= -80.........ans.
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