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Given that 1st term of an A.P is 7 and it's 10th term is twice the second term,calculate the (a) 19th term (b)sum of 28 terms (c) difference between 9th and 6th terms

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a10=a1+r(10-1)
14=7+r(9)
r=7/9
check
7, 7 7/9, 7 14/9, 7 21/9, 7 28/9, 7 35/9, 7 42/9, 7 49/9, 7 56/9, 14

Use the formulas for sum, knowing r, a1

1st term=7 2nd term=a+d
10term=a+9d
7+9d=2(14+2d)
14-7=9d-2d
7=7d=d=1
sum of 28 term
n/2(2a+(n-1)d)
28/2(2*7+(28-1)1)
14(14+27)=14(41)
25term=574
9th term=7+(9-1)1
7+(9-1)
7+8=15
6th term=7+(6-1)1
7+(6-1)=7+5=12
9th term-6th term=15-12=3.

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