Given two moving points A(x1,y1) and B(x2,y2) on parabola curve y2=6x with x1+x2=4 and x1=x2, and the perpendicular bisector of segment AB intersects x-axis at point C. Find the maximum area of △ABC.
Solutions — 2
Solution 1
Let the midpoint of AB be M(x0,y0). Then x0=2x1+x2=2 and y0=2y1+y2. We have kAB=x2−x1y2−y1=6y2+6y16y2−6y1=y26+y16y26+y16=y03. The equation of the perpendicular bisector of AB is y−y0=−3y0(x−2).1◯ It is easy to find that one solution of it is x=5, y=0. Therefore, the intersection C is a fixed point with coordinate (5,0). From ①, we know the equation of line AB is y−y0=y03(x−2), or x=3y0(y−y0)+2.2◯ Substituting ② in y2=6x, we get y2=2y0(y−y0)+12, or y2−2y0y+2y02−12=0.3◯ As y1 and y2 are two real roots of ③ and y1=y2, we have Δ=4y02−4(2y02−12)=−4y02+48>0. Therefore, −23<y0<23. Then we have The distance from point C(5,0) to segment AB is h=∣CM∣=(5−2)2+(0−y0)2=9+y02. Therefore, S△ABC=21∣AB∣⋅h=31(9+y02)(12−y02)⋅9+y02=3121(9+y02)(24−2y02)(9+y02)≤3121(39+y02+24−2y02+9+y02)3=3147. The equality holds if and only if 9+y02=24−2y02, i.e. y0=±5. Then we get andA(36+35,5+7),B(36−35,5−7) A(36+35,−(5+7)),B(36−35,−5+7).
Solution 2
Similar to Solution 1, we get that C, the intersection of the perpendicular bisector of AB and the x-axis, is a fixed point with coordinate (5,0). Let x1=t12, x2=t22, t1>t2, t12+t22=4. Then S△ABC is the absolute value of 215t12t2206t16t2111, so S△ABC2=(21(56t1+6t12t2−6t1t22−56t2))2=23(t1−t2)2(t1t2+5)2=23(4−2t1t2)(t1t2+5)(t1t2+5)≤23(314)3. Therefore, S△ABC≤3147 and the equality holds if and only if (t1−t2)2=t1t2+5 and t12+t22=4. We then get t1=67+5 and t2=−67−5, which implies either A(36+35,5+7),B(36−35,5−7) or A(36+35,−(5+7)),B(36−35,−5+7).
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