Two APs have the same first and last terms. The first AP has 21 terms with a common difference is 9. How many terms has the other AP if it's common difference is 4
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a and d is common i.e a1 =a2
tn1 = tn2
t21 = a + (21-1) ×9
= a + (20×9)
= a + (180)
tn2 = a+ (n-1) 4
a + (180) = a + (n-1)×4
180 = 4n - 4
4n = 176
n = 176/4
n = 44
the other AP has 44 terms
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