Question

已知A=5a+3b，B=3a²-2a²b，C=a²+7a²b-2，當a=1，b=2時，求A-2B+3C的值．

Answer 1

∵A=5a+3b，B=3a²-2a²b，C=a²+7a²b-2，
∴A-2B+3C=（5a+3b）-2（3a²-2a²b）+3（a²+7a²b-2）
=5a+3b-6a²+4a²b+3a²+21a²b-6
=-3a²+25a²b+5a+3b-6，
當a=1，b=2時，原式=-3×1²+25×1²×2+5×1+3×2-6=52．