Find the sum of first seventy terms of an AP whose 4th and 9th term are -15 and -30
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a=?
d=?
But we know
a+3d=-15
a+8d=-30
By elimination method
-5d =15
d=-3
a=-6
Sum =70/2(2a+(n-1) d)
35×(-12+69×-3)
-7665
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