If the sum of 1st 4 terms of an ap is 40 and that of 1st 14 terms is 280. Find the sum of 1st n terms.
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Step-by-step explanation:
T4=4/2(2a+3d)
40=2(2a+3d)
20=2a+3d ---(1)
T14=14/2(2a+13d)
280=7(2a+13d)
40=2a+13d ---(2)
From eqn (1) and (2)
40-20= 2a-2a+13d-3d
20=10d
d=2
a=7
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