Algebra
posted by G on .
Fomula: A=1/2(b1+b2)h
A=area of a trapezoid
b1=width at the top
b2width at the bottom
h=height
A 7foor tall trapezoid has 42 square feet of area. If is is 9 feet wide at the bottom, how far across is it at the top?

I will set it up for you and you can simplify.
A = 42
h = 7
b2 = 9
I will plug and you can chug.
42 = 1/2(b1 + 9)(7)
Solve for b1, which is read "b sub one."
Can you take it from here? 
Thank you for your response. I can set up and calculate. The book says I should be getting 3 feet as my answer. That is not what I am getting.

Here it is:
First multiply 7 x 1/2 = 7/2.
We now have this equation:
42 = 7/2(b1) + 9)
Simplify the right side of the equation and the whole thing becomes
42 = 7/2(b1) + 63/2
Subtract 63/2 from 42.
42  63/2 = 10_1/2
We now have:
10_1/2 = 7/2(b1)
10_1/2 divided by 7/2 = b1
3 feet = b1
Done!