a,2x + 5 = x – 1 b,1 – 2 – 3 + 4 + 5 – 6 – 7 + 8 + …. + 801 – 802 – 803 + 804

a,2x+5-x+1=0
⇔x+6=0
⇔x=-6
b, 1-2-3+4+5-6-7+8+9-10-…+801-802-803+804

=1+(-2+4)+(-3+5)+(-6+8)+(-7+9)+…+(-799+801)+(-802+804)-803