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Algebra Difficulty 4.9 AIME Find the answer Philippines

Problem:
The operation * satisfies the following properties:
x0=0,x(y+1)=xy+(xy) x * 0=0,\quad x *(y+1)=x * y+(x-y)
Evaluate 2010102010 * 10.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:
Let us compute 2010102010 * 10 using the given properties.

Let f(y)=2010yf(y) = 2010 * y.

We are given:
f(0)=0 f(0) = 0
f(y+1)=f(y)+(2010y) f(y+1) = f(y) + (2010 - y)
Let us compute step by step:

f(0)=0f(0) = 0

f(1)=f(0)+(20100)=0+2010=2010f(1) = f(0) + (2010 - 0) = 0 + 2010 = 2010

f(2)=f(1)+(20101)=2010+2009=4019f(2) = f(1) + (2010 - 1) = 2010 + 2009 = 4019

f(3)=f(2)+(20102)=4019+2008=6027f(3) = f(2) + (2010 - 2) = 4019 + 2008 = 6027

f(4)=f(3)+(20103)=6027+2007=8034f(4) = f(3) + (2010 - 3) = 6027 + 2007 = 8034

f(5)=f(4)+(20104)=8034+2006=10040f(5) = f(4) + (2010 - 4) = 8034 + 2006 = 10040

f(6)=f(5)+(20105)=10040+2005=12045f(6) = f(5) + (2010 - 5) = 10040 + 2005 = 12045

f(7)=f(6)+(20106)=12045+2004=14049f(7) = f(6) + (2010 - 6) = 12045 + 2004 = 14049

f(8)=f(7)+(20107)=14049+2003=16052f(8) = f(7) + (2010 - 7) = 14049 + 2003 = 16052

f(9)=f(8)+(20108)=16052+2002=18054f(9) = f(8) + (2010 - 8) = 16052 + 2002 = 18054

f(10)=f(9)+(20109)=18054+2001=20055f(10) = f(9) + (2010 - 9) = 18054 + 2001 = 20055

Therefore, 201010=200552010 * 10 = 20055.

Alternatively, we can find a closed formula.

Let us try to find a general formula for xyx * y.

Let f(y)=xyf(y) = x * y.

We have:
f(0)=0 f(0) = 0
f(y+1)=f(y)+(xy) f(y+1) = f(y) + (x - y)
So,
f(1)=f(0)+(x0)=x f(1) = f(0) + (x - 0) = x
f(2)=f(1)+(x1)=x+(x1)=2x1 f(2) = f(1) + (x - 1) = x + (x - 1) = 2x - 1
f(3)=f(2)+(x2)=(2x1)+(x2)=3x3 f(3) = f(2) + (x - 2) = (2x - 1) + (x - 2) = 3x - 3
f(4)=f(3)+(x3)=(3x3)+(x3)=4x6 f(4) = f(3) + (x - 3) = (3x - 3) + (x - 3) = 4x - 6
So, the pattern is:
f(y)=yxy(y1)2 f(y) = yx - \frac{y(y-1)}{2}
Let us check for y=4y=4:
f(4)=4x4×32=4x6 f(4) = 4x - \frac{4 \times 3}{2} = 4x - 6
Correct.

Therefore,
201010=10×201010×92=2010045=20055 2010 * 10 = 10 \times 2010 - \frac{10 \times 9}{2} = 20100 - 45 = 20055

Final answer:
20055 \boxed{20055}

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.