This maximum/minimum proboblem (of a parabola) is from 8E of the Y11 Cambridge 3U book (p298):
14. A Tasmanian orchardist notices that an apple tree will produce 300 apples per year if 16 trees are planted in every standard-sized field. For every additional tree planted in the standard-sized field, she finds that the yield per tree decreases by 10 apples per year.
a) If she plants an additional x trees in every standard-sized field, show that the total number of apples produced will be:
N = -10x2 + 140x + 4800
14. A Tasmanian orchardist notices that an apple tree will produce 300 apples per year if 16 trees are planted in every standard-sized field. For every additional tree planted in the standard-sized field, she finds that the yield per tree decreases by 10 apples per year.
a) If she plants an additional x trees in every standard-sized field, show that the total number of apples produced will be:
N = -10x2 + 140x + 4800