已知a^2-4ab=21,3ab+b^2=7,求整式3a^2+4b^2的值

题目

已知a^2-4ab=21,3ab+b^2=7,求整式3a^2+4b^2的值

答案

3a²+4b²
=3(a²-4ab)+4(3ab+b²)
=3*21+4*7
=63+28
=91
x³+3x²+1
=x(x²+x+2x)+1
=x(2+2x)+1
=2(x²+x)+1
=2*2+1
=5