Question

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Answer 1

$a = 7 \\ d = a2 - a1 \\ d = 11 - 7 \\ \\ d = 4 \\ \\ n = 20 \\ \\ sn = \frac{n}{2} (2a + (n - 1)d) \\ \\ s20 = \frac{20}{2} (2 \times 7 + (20 - 1)4) \\ \\ s20 = 10(14 + (19)4 \\ \\ s20 = 10(14 + 76) \\ \\ s20 = 10(90) \\ \\ s20 = 900$

Step-by-step explanation:

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Answer 2

Step-by-step explanation:

AP : 7, 11, 15, 19, . . .

a = 7

d = 4

n = 20

Sn = n/2 { 2a + ( n-1)d}

= 20/2 { 2×7 + ( 20-1)4}

= 10 { 14 + (19)4}

= 10 { 14 + 76 }

= 10 { 90}

= 900

Thus, sum of 20 terms = 900

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