Question

if tn=3-4n,find s20
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Answer 1

Step-by-step explanation:

t1=-1

t2=-5

t3=-9

t4=-13

s20=20/2[2(-1)+19(-4)]=10(-2-76)=-780

Answer 2

The sum of first 20 term is -780.

Step-by-step explanation:

Consider the provided information.

$t_n=3-4n$

Substitute n=1 in $t_n=3-4n$

$t_1=3-4(1)$

$t_1=-1$

Thus, the first term of A.P is -1.

Substitute n = 2 in $t_n=3-4n$

$t_2=3-4(2)$

$t_2=3-8$

$t_2=-5$

The common difference is $d=t_2-t_1$.

$d=-5-(-1)$

$d=-4$

To calculate the sum of first 20 terms use the formula: $s_n=\frac{n}{2}(2a+(n-1)d)$

$s_{20}=\frac{20}{2}(2(-1)+(20-1)(-4))$

$s_{20}=10(-2+(19)(-4))$

$s_{20}=10(-2-76)$

$s_{20}=10(-78)$

$s_{20}=-780$

Hence, the sum of first 20 term is -780.

#Learn more

If tn = 3+4n then find AP and hence find its 20th term.

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