1) P = \(3+15x-5x^2\)\(=-5x^2+15x+3=-5\left(x^2-3x-\frac{3}{5}\right)\) \(=-5\left(x^2-2.\frac{3}{2}.x+\frac{9}{4}-\frac{9}{4}-\frac{3}{5}\right)\)= \(-5\left[\left(x-\frac{3}{2}\right)^2-\frac{57}{20}\right]=-5.\left(x-\frac{3}{2}\right)^2+\frac{57}{4}\)
vì \(\left(x-\frac{3}{2}\right)^2>=0\) => \(-5.\left(x-\frac{3}{2}\right)^2+\frac{57}{4}>=0\) =>\(-5.\left(x-\frac{3}{2}\right)^2+\frac{57}{4}>=\frac{57}{4}\)
=> GTLN của P là \(\frac{57}{4}\)tại x =\(\frac{3}{2}\)
2) GTNN của B là -36