1.

The sum of the squares of the fifth and the eleventh term of an AP is 3 and the product of the second and the fourteenth term is equal it P. Find the product of the first and the fifteenth term of the AP. (a) (58P – 39)/45 (b) (98P + 39)/72 (c) (116P – 39)/90 (d) (98P + 39)/90

Answer»

Correct option (c) (116P – 39)/90    

Explanation: 

(a + 4d)2 + (a + 10d)2 = 3 → a2 + 14ad + 58d2 = 1.5. 

Also, (a + d)(a + 13d) = P → a2 + 14ad + 13d2 = P. Further, we need to find the value of a2 + 14ad (product of the first and fifteenth terms of the AP). From the above two equations, we get that 45d2 =3/2-p →13d2 =(39-26p)/90

Thus, a2 + 14ad = P −(39-26p)/90 = (116P − 39)/90.



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