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Question

If a1,a2,a3,.... are in A.P. such that a1,a5,a10+a15+a20+a24=225, then a1+a2+a3+...+a24 is equal to

A
600
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B
900
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C
1200
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D
1800
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Solution

The correct option is B 900
a1+a5+a10+a15+a20+a24=225 (given)
a+(a+4d)+(a+9d)+(a+14d)+(a+19d)+(a+23d)=225
6a+69d=225...(i)
a1,a2,a3... in A.P
a1a2=a+d,a3=a+2d etc
a1+a2+a3+....+a24=?
n=24,S24=n2[2a+(n1)d]=242[2a+23d]
=12[75] [from eq. (i)]
= 900 Ans.
from eq. (i)
3[2a+23d]=225
2a+23d=75...(ii)

1162306_1277706_ans_bd2c71bcd5b64d798d8d7ab81edfba81.jpg

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