第1个回答 2018-05-19
分别以CB,CA为x,y轴建立直角坐标系,则M(10,0),D(0,6),设P(x,y),满足
x^2+y^2=144,①
微分得2xdx+2ydy=0,
∴dy=-xdx/y,②
w=DP+PM/2=√[x^2+(y-6)^2]+(1/2)√[(x-10)^2+y^2]
=√(x^2+y^2-12y+36)+(1/2)√(x^2-20x+100+y^2)
=√(180-12y)+(1/2)√(244-20x)(由①)
=2√(45-3y)+√(61-5x),(*)
∴dw=-3dy/√(45-3y)-5dx/[2√(61-5x)]
=3xdx/[y√(45-3y)]-5dx/[2√(61-5x)],(由②),
由dw/dx=0,得6x√(61-5x)=5y√(45-3y),③
平方得36x^2(61-5x)=25y^2(45-3y),
两边都除以3,代入y^2=144-x^2,得
12x^2(61-5x)=25(144-x^2)(15-y),
∴y=15-12x^2(61-5x)/[25(144-x^2)],④
代入①,x^2+{15-12x^2(61-5x)/[25(144-x^2)]}^2=144,
去分母得625x^2(144-x^2)^2+[375(144-x^2)-12x^2(61-5x)]^2=90000(144-x^2)^2,
625x^2(144-x^2)^2+(54000-1107x^2+60x^3)^2=90000(144-x^2)^2,分离系数得
625..................-180000.....................12960000
3600..-132840..1225449..6480000...-119556000.............2916000000
.......................-90000....................25920000..............-1866240000
4225x^6-132840x^5+955449x^4+6480000x^3-80676000x^2+1049760000=0,
解得x1≈11.469807,x2≈5.57161285,或
f(x)=4225x^4-60840x^3-351351x^2+4380480x+16426800≈0,
f’(x)=16900x^3-182520x^2-702702x+4380480,
≈16900(x-3.80144387919)(x-12.46753098)(x+5.468975011),
X<-5.4或3.8<x<12.4时f’(x)<0,f(x)是减函数;此外,f(x)是增函数。
f(12.46753098)≈604105,是f(x)的极小值,所以在0至12.4,f(x)无零点。
分别代入④,y1≈-3.52753403(不合③,舍去),y2≈10.6281292,
代入(*),得w≈13,为所求。