Question 983043
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Using expression for general term of the sequence, A for first term, when n=1,
{{{A+(n-1)d}}};


The description will give this system:
{{{system(A+A+(n-1)d=42,(n/2)(A+A+(n-1)d)=420,A+d=4)}}}


Some simplification,
{{{system(2A+nd-d=42,(n/2)(2A+nd-d)=420,A+d=4)}}}


{{{system(2A+nd-d=42,n(2A+nd-d)=840,d=4-A)}}}


Try substituting for d in the first two equations of this last form of the system....


...  Working through the algebra steps for that brings to the system:
{{{system(3A+4n-nA-4=42,n(3A+4n-nA-4)=840)}}}


Dividing the second equation by the first equation gives
(because notice the four-term polynomial expression in both equations)
{{{n=840/42}}}
{{{highlight(n=20)}}}


Review everything and try to understand, and then continue to find A and d.