A5+a9=40
a8+a14=64
find the sum of first 20 terms
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Step-by-step explanation:
a+4d+a+8d =40 , a+7d+a+13d = 64
2a + 12d = 40 , 2a + 20d = 64
a + 6d = 20----(1) a + 10d = 32----(2)
from equation (2) - (1)
4d = 12 , d = 3
a = 20 - 6(3) = 20-18 = 2
S(20) = 20/2 {2a + 19d}
= 10 {2a + 19d}
= 10 { 4 + 57 } = 10×61 = 610
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