Sum: a+b/2,a,3a-b/2,.... to 21 terms

these are in ap.

common difference(d)=a-b/2

first term(A)=a+b/2

number of terms(n)=21

sum=n/2[2a+(n-1)d]

21/2[2*(a+b/2)+(21-1)(a-b)/2]

sum=21/2[11a-9b]

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