Question

find the sum of the first 20 terms of the AP 16, 11, 6, 1, - 4​

Answer 1

a=16,d=11-16=-5,n=20

S20=n/2(2a+(n-1)d)

=20/2(2×16+(20-1)×-5)

=10(32+(-95))

=10(-63)

=-630

Answer 2

$\Rightarrow\Huge\boxed{Given}$

AP = 16 , 11 , 6 , 1 , -1.

so,

a = 16.

d = -5.

A/Q,

$sn = \frac{n}{2} .\: \: {2a + (n - 1)}d . \\ \\ \: \: \: \: \: \: = \cancel \frac{20}{2} (32 - 95). \\ \\ \: \: \: \: \: \: = 10( - 63). \\ \\ \: \: \: \: \: \: = - 630.$

$\huge\underline\mathfrak\purple{Answer}$

= -630.

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