a=7,a13=32 find d and s13
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Step-by-step explanation:
an=a+{n-1}d
a13=7+(13-1)d
35=7+12d
35-7=12d
28=12d
d=28÷12
by cut and short method
d=7÷3-______ Equation 1
An=n÷2{2a+(n-1)d}______equation 2
By applying equation 1 in 2 we get
S13=13÷2{2×7+(13-1)7÷3
=13÷2{14+. 12×7÷3}
=13÷2(14+4×7) { we cut 12 by 3 }
=13÷2(42)
=13×21. {we cut 42 by 2}
=273
So d=7÷3.and S13=273
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