a^2-2ab=3b^2-ab=4 由式2乘以2减式1，得 2b^2-2ab-a^2+2ab=2*4-3 即 2b^2-a^2=5
a^2-2ab=3,b^2-ab=42b^2-a^2=8+2ab-(3+2ab)=5
