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AD.1 [latex]left { {{a6=a1+5r} atop {a10=a1+9r}} ight.[/latex] [latex]left { {{20=a1+5r} atop {4=a1+9r}} ight.[/latex] [latex]left { {{-20=-a1-5r} atop {4=a1+9r}} ight.[/latex] [latex]-20+4=-a1+a1-5r+9r [/latex] [latex]-16=4r[/latex] [latex]r=-4[/latex]   [latex]4=a1+9*(-4)[/latex] [latex]4=a1-36[/latex] [latex]a1=40[/latex]