SAT Math · Nonlinear equations in one variable · medium

The equation 2^x = x + 5 can be solved by finding the x-coordinates where the graphs of y=2^x and y=x+5 intersect. How many real solutions…

The equation 2x=x+52^x = x + 5 can be solved by finding the xx-coordinates where the graphs of y=2xy=2^x and y=x+5y=x+5 intersect. How many real solutions does this equation have?

Answer and explanation

Answer: A: 2

Compare the graphs of y=2xy = 2^x and y=x+5y = x + 5. At x=3x = 3, both equal 8, so one intersection is (3,8)(3, 8). For large xx, the exponential grows faster than the line, so there are no more intersections to the right. For negative xx, the line y=x+5y = x + 5 decreases through zero at x=−5x = -5, while 2x2^x stays positive and small; at x=−5x = -5 the exponential is above the line (132>0\frac{1}{32} > 0), but at x=−4x = -4 the line is above (1>1161 > \frac{1}{16}). So they cross once between −5-5 and −4-4. That is 2 solutions, choice A.

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