Question

the sum of first 20 terms of an ap is 400 and the sum of first 40 term is 1600 find the sum of first 10

Answer 1

Answer:

Step-by-step explanation:

S20=400

S20=20/2[2a+[20-1]d]

400=20a+190d

40=2a+19d

S40=1600

S40=40/2{2a+[40-1]d}

1600=20{2a+39d}

80=2a+39d

Answer 2

Answer:

Step-by-step explanation:S20=400=20×20

S40=1600=40×40

S10=10×10=100