solve the solution 1+4+7+10.............+x = 287

x = 42

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1 + 4 + 7 + 10  + .......................... + x = 287
Let there be n terms in the A.P

so, n/2 [2(1) + (n - 1)(3)] = 287

=> n(2 + 3n - 3) = 574

=> 3n² - n - 574 = 0

=> n = (1+83)/6 or n = (1 - 83)/6

ignoring the -ve value we get

n = 14

so, there are 14 terms and x is the 14th term

so, x = 1 + 13(3) = 1 + 39 = 40

clearly, x is the 14th term and the value of x is 40 Ans...

ans is 40