for an A.P, if t4=20 and t15=64, find tn
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formula to calculate tn= a+(n-1)d
it is given that
t4= a+3d = 20
t15= a+14d = 64
subtracting t15-t4 we get
a+14d-(a+3d)
a+14d-a-3d
11d=44
d= 44/11 = 4
a+3d=20
putting d=4. we get,
a+3×4 =20
a+12=20
a=8
thus tn= 8+(n-1)4
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