Question

Find the sum of first 24 terms of an

a.p. a1,a2,a3,...,if it is known that a1 + a5 + a10 + a15 + a20 + a23 = 225

Answer 1

Given,

a1+a5+a10+a15+a20+a23 = 225
a+ (a+4d) + (a+9d) + (a+14d) + (a+19d) + (a+22d) = 225
6a+69d = 225

Here, Sn = n/2(a+l)
So,
S24 = 24/2(a+(a+23d))
        = 12* (2a+23d)
        =  12* (225/3)
        =   4*225
        =   900

Answer 2

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