Math Help!!
posted by Andrew .
Find all points of the graph of y=7x^2+7x whose tangent lines are perpendicular to the line 49y=x

y = 1/49 x
the slope of the perpendicular is
... (1/49) = 49
the 1st derivative is the slope of the tangent ... dy/dx = 14x + 7
14x + 7 = 49
find x, then find y from the original function 
49y=x has slope 1/49
so, you want lines with slope 49
since y' = 7(2x+1), that means x=3.
y(3) = 84
so, the line is
y84 = 49(x3)
See
http://www.wolframalpha.com/input/?i=plot+y%3D7x%5E2%2B7x,+y%3D49(x3)%2B84+for+x%3D0..5
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