#MAASL00011/#MAISL00042


a) u_{3}=u_{1}+(n-1)d

12 = 2+ (2) d

10 = 2d

 d=5



b) u_{20} = 2+ (19)5

 u_{20}=97



c) S_{20} = n(\frac{a_{1}+a_{n}}{2})

S_{20} = 20(\frac{2+97}{2}) 

S_{20}=990

Explanation

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