21. Find an equation of the tangent plane to the surface x2 + 2y2 + 322 = 6 at the point (1, 1, -1). A. -x + 2y + 3z =2 B. 2.x + 4y - 6z = 6 C. x - 2y + 3z = -4 D. 2.x + 4y - 6z = 0 E. x + 2y - 3z = 6. 22. Find an equation of the plane tangent to the graph of f(x, y) = 7 + sin(x2 + 2y) when (x, y) = (2, T). A. 47x + 2y - 2 = 97 B. 4x + 2Ty - 2 = 10T C. 4rx + 2xy + 2 = 10n D. 4x + 2Ty - z = 97 E. 4xx + 2y + z = 9T. 3 23. The differential df of the function f(x, y, z) = xey2-z2 j A. df = xev -z dx + xey?-2 dy + xey? -z dz B. df = xeyz -22 -z d.x dy dz C. df = ey"-* dax - 2xyey?-z dy + 2xzey2-z2 dz D. df = ey-" dax + 2xyey"-zdy - 2xzey2-z2 dz E. df = ey- ->(1+2xy - 2.cz)