Curve is Y= 4x +3 cos2x. The normal to the curve when x= pi/4 meets the x-and y-axis at point A and B . Find the area of the triangle AOB  O=(0,0) 

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Borys Shumyatskiy | College Teacher | (Level 3) Associate Educator

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The area of the (right) triangle AOB is AO*OB/2. We have to find the equation of normal and the points A and B, its x- and y- intercepts.

y(x) = 4x + 3cos(2x).
Denote x0 = pi/4, y0 = y(x0) = pi + 3cos(pi/2) = pi.

Now find the equation of the normal.

1. It passes the point (x0, y0).

2. The slope of the tangent line at the point (x0, y0) is y'(x0).
y'(x) = 4 - 6sin(2x), y'(x0) = 4 - 6sin(pi/2) = -2.

So the slope of the normal is -1/(-2) = 1/2 and the equation of the normal is

y-y0 = (1/2)*(x-x0), or y-pi = (1/2)(x-pi/4).

For the point A put y=0 and obtain x = 2*(-pi) + pi/4 = -(7/4)pi. So AO = (7/4)pi.

For the point B put x=0 and obtain y = pi - pi/8 = (7/8)pi. So OB = (7/8)pi.

At last, the area = (1/2)*(7/4)pi*(7/8)pi = (49/64)pi^2. It is approx.= 7.556.

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