Answer: 15, -8
Step-by-step explanation:
[tex]u_2=7\\u_2=u_1+r\\u_3=u_1+2r\\u_4=u_1+3r\\\\u_1+u_2+u_3+u_4=4u_1+6r=12\\u_1+r=7\\\\[/tex]
[tex]\left \{ \begin {array} {ccc|c|c}u_1+r&=&7&-4&-6\\4u_1+6r&=&12&1&1\\\end {array} \right.\\\\\\\left \{ \begin {array} {ccc}2r&=&-16\\-2u_1&=&-30\\\end {array} \right.\\\\\\\left \{ \begin {array} {ccc}r&=&-8\\u_1&=&15\\\end {array} \right.\\\\\\Proof:\\u_1=15\\u_2=7\\u_3=-1\\u_4=-9\\sum=12\\[/tex]
