57. sn=(2a₁+d*(n-1))*n/2;
400=(2*4+8*(n-1))*n/2;
400=(4+4*(n-1))*n; 400=4n²; n=±10, т.к. n∈N, то n=10
58. sn=(a₁+an)*n/2; sn=(5+599)*100/2=60400/2=30200;
a₁₀₀=a₁+99d=599; 99d=599-5; d=594/99=6
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57. sn=(2a₁+d*(n-1))*n/2;
400=(2*4+8*(n-1))*n/2;
400=(4+4*(n-1))*n; 400=4n²; n=±10, т.к. n∈N, то n=10
58. sn=(a₁+an)*n/2; sn=(5+599)*100/2=60400/2=30200;
a₁₀₀=a₁+99d=599; 99d=599-5; d=594/99=6