a, b, c and d are four numbers which are in A.P. and relation between a, b and d is
a + 3b - d = 6 and between c and d is c - d = -4. Then find the sum till 60 terms of
the A.P [Ans. 7200]
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1 Answers
1since a, b,c and d are in AP
we 've....
a + c = 2b ........ (1)
b + d = 2c ..........(2)
given that....
a + 3b - d = 6 ....... (3)
c - d = -4 .......... (4)
Solving the 4 equations... we get..
a = 2 , b= 6 , c= 10 , d=10
therefore...
S60 = n/2[2a + (n-1)diff]
S60 = 30 (2 x 2 + 59 x 4)
S60 = 30 x 240 = 7200
