the sum of first 9 term of a.p.is 351 and the sum of its first 20 terms is 1770.find the first and common difference of the a.p.
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s9 = 351
9/2(2a +8d) = 351
9(a+4d) = 351
a+4d = 39...... 1️⃣
also...
s20 = 1770
20/2(2a + 19d) = 1770
2a+9d = 177.......2️⃣
multiply eq.1 by 2 and then subtract 1 from 2
d= 99 then a is -357
A.P. is - 357, - 258
9/2(2a +8d) = 351
9(a+4d) = 351
a+4d = 39...... 1️⃣
also...
s20 = 1770
20/2(2a + 19d) = 1770
2a+9d = 177.......2️⃣
multiply eq.1 by 2 and then subtract 1 from 2
d= 99 then a is -357
A.P. is - 357, - 258
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