1.three numbers are in AP and their sum is 15. If 8,6,4 are added to them respectively,they form a GP.The numbers are.
Please tell me the solution and method of solving this type of questions using this formula for Ap:(a-d),a, (a+d) and for GP:a/r,a,ar
The option for the above question is :A)2,6,7 B)4,6,5 C) 3,5,7

The above is question of AP and GP.

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Formula for the nth term of an AP:

T_n = a + (n-1)d

The sum of n terms in an AP is:
S_n = \frac12 (a+l) = \frac12 (2a + (n-1)d)

Those fot a GP;
T_n = ar^{(n-1)}

S_n = \frac{a(1-r^n)}{1-r} \ for\ r\cancel{=} 1

S_n = \frac{a}{1-r} \ for\ |r|<1

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