Question

plz solve this...




no irrelevant answers plz​

Answer 1

Answer:

sorry

Step-by-step explanation:

pic is all black sorry sorry sorry

Answer 2

Answer:

• First term (a) = -18
• Common difference (d) = 6
• a(18) = 84

Step-by-step explanation:

Given:

• Sum of first 20 terms (1-20) of an AP = 780
• Sum of next 20 terms (21-40) = 3180

To find:

• First term (a)
• Common difference (d)
• 18th term (a(18))

Required Formulae:

1. Term formula : nth term of an AP

a(n) = a + ( n - 1) d

where,

• a = first term of an AP
• d = common difference

2. Sum formula : sum of n terms of an AP

S = n/2 * ( 2a + (n-1)d )

where,

• a = first term of an AP
• d = common difference

Solution:

S(20) = 780

=> 20/2 * ( 2a + (20 - 1) d ) = 780

=> 10 ( 2a + 19d ) = 780

=> 2a + 19d = 78

2a + 19d = 78 (Equation 1)

Sum of next 20 terms = 3180

Here, first term will be 21st term of AP and difference will be the same

which means a(new) = a + ( 21 - 1 ) d = a + 20d

S = 20/2 * ( 2a(new) + (20 - 1) d )

=> 10 * ( 2(a + 20d) + 19d ) = 3180

=> 10 * ( 2a + 40d + 19d ) = 3180

=> 2a + 59d = 318

2a + 59d = 318 (Equation 2)

Subtracting equation 1 from equation 2, we get

2a + 59d - (2a + 19d) = 318 - 78

=> 40d = 240

=> d = 6

d = 6

Putting d = 6 in equation 1, we get

2a + 19(6) = 78

=> 2a + 114 = 78

=> 2a = 78 - 114 = -36

=> a = -18

a = -18

a(18) = a + ( 18 - 1 ) d = a + 17d

= -18 + 17(6)

= -18 + 102

= 84

a(18) = 84