# Calculus

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How many terms are required in the series (-6) + (-12) + (-24)... to add to a sum of -378?

How do I do this?

• Calculus -

looks like a geometric series where
a = -6 and r = 2
Term(n) = a(r^n - 1)/(r-1), where n is the number of terms
-378 = -6(2^n - 1)/(2-1)
63 = 2^n - 1
64 = 2^n
I know that 2^6 = 64
so n=6

so you need 6 terms.

• Calculus -

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