sum(8) = 4(2a + 7d) = 100
2a + 7d = 25 ***
sum(20) = 10(2a + 19d) = 880
2a + 19d = 88 ***
subtract them:
12d = 63
d = 21/4
in ***
2a + 7(21/4) = 25
2a = -47/4
a = -47/8
sum12 = 6(2(-47/8) + 11(21/4) = 276
The sum of 8 term of an a-p is 100 while the sum of 20 term is 880 find the sum of 12 term
1 answer
1 answer