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Sagot :
Answer:
S_6 = 252
Step-by-step explanation:
We are told that the 10th, 4th and 1st term of an A.P are three consecutive terms of a G.P.
Now,formula for nth term of an AP is;
a_n = a + (n - 1)d
Thus;
a_10 = a + (10 - 1)d
a_10 = a + 9d
Also;
a_4 = a + (4 - 1)d
a_4 = a + 3d
First term is a.
Thus, since they are consecutive terms of a G.P, it means that;
(a + 9d)/(a + 3d) = (a + 3d)/a
Cross multiply to get;
a(a + 9d) = (a + 3d)(a + 3d)
a² + 9ad = a² + 6ad + 9d²
a² will cancel out to give;
9ad - 6ad = 9d²
3ad = 9d²
Divide both sides by 3d to get;
a = 3d
We are told that the first term is 4.
Thus, 4 = 3d
d = 4/3
We saw earlier that ratio of the GP is (a + 3d)/a
Thus; r = (4 + 3(4/3))/4 = 8/4 = 2
Sum of n terms of a G.P is given by;
S_n = a(rⁿ - 1)/(r - 1)
S_6 = 4(2^(6) - 1)/(2 - 1)
S_6 = 252
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