Question

find the sum of first 20th terms of ap 12,7,12.........using the formula s=n/2(2a+(n-1) d) ​

Answer 1

Step-by-step explanation:

Given,

AP is 2,7,12,17......

where a = 2

d =5

n = 20

Now we know that \implies⟹

Sn = \dfrac{n}{2}

2

n

[2a + (n-1)d]

=> Sn = \dfrac{20}{2}

2

20

[2*2 + (20-1)5]

=> Sn = 10[4 + 19*5]

=>Sn = 10(4 + 95)

=>Sn = 10 * 99

=> Sn = 990.

Hence the sum of 20 terms of the AP is 990.

Remember

1)Sn = \dfrac{n}{2}

2

n

[2a + (n-1)d]

-For finding sum till nth term of an AP

2)An = 2a + {n-1}d

-For finding nth term of an AP.

Answer 2

Step-by-step explanation:

a = 12

d = a2 - a1 = 7 - 12 = -5

on using AP formula

s= 20/2 ( 2×12 + ( 20 - 1 ) d )

s = 10 ( 24 + 19 × -5 )

s = 10 ( 24 + -95)

s = 10 × - 71

s = - 710

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