What is the sum of first 15 terms of an ap whose 11th and 7th terms are 5.25 and 3.25?
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a+10d=5.25. (1)
a+6d=3.25. (2)
solving (1)&(2)
d=0.5
a=0.25
S15=56.25
a+6d=3.25. (2)
solving (1)&(2)
d=0.5
a=0.25
S15=56.25
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