一道数学jj问题？

sylvia

有一点A(r,o), 一点B(s,t), 且rst相乘不等于零,问三角形OAB面积?
1)    r=3
2)    t=-6
【讨论】 c
我觉得是E.
大家的意见和解法是什么呢？

lisaislisa

here is the version I encountered
DS: 有一线平行于X=Y, 不在II象限,有一点A(r,o), 一点B(s,t), 且rst相乘不等于零,问三角形OAB面积?
1)    r=3
2)    t=-6
my An:B

^^

uclb5550

如果题意完整的话， 我个人认为应该是E,since we have no idea about S.

lisaislisa

but how about the version with 'parallel x=y?' ?
no difference ?

^^

uclb5550

不明白为什么选B, 不知道S的值，你怎么确定这个三角形的形状呢？

uclb5550

X=Y is a straight slant line evenly divided the first and third quadrant.(certainly if it is not on the second quadrant) the line parralleled it will also acrosses the first, third and fourth quadrant. but it saies nothing about the shape of triangle OAB.
if A and B are two points on the line and triangle OAB indicates the triangle shaped by A, B and center point O, then the answer would be C.

area triangleOAB= 3*6/2

uclb5550

sorry, " but it saies nothing about the shape of triangle OAB." this sentence should be moved.

vincentsam

UCLB5550
我同意这题是C,但是好多人说是B,搞不懂

uclb5550

alright, i see. the answer should be B. No problem. since that line parralels X=Y, the piont that line acrosses ordinate must in the middle point of 6 and is equal with r since this triangle is isosceles triangle. therefore, if we know t=6, then r must be 3. good! thanks for the remind from you and lisaislisa!

sylvia

我怎么还是不明白。看起来是C的。怎么会是B?
题目的意思是AB平行于x=y,也就是t=s-r
而面积S= 1/2 * r*t .(r与t没有必然关系的阿） 比如划一条线和x轴正半轴交45度，可以平行的画若干条的t都相同而r不同阿