We consider skew tent maps $ T_{ { \alpha }, { \beta }}(x) $ such that $ ( \alpha, \beta)\in[0, 1]^{2} $ is the turning point of $ T _{\alpha, \beta} $, that is, $ T_{ { \alpha }, { \beta }} = \frac{ { \beta }}{ { \alpha }}x $ for $ 0\leq x \leq { \alpha } $ and $ T_{ { \alpha }, { \beta }}(x) = \frac{ { \beta }}{1- { \alpha }}(1-x) $ for $ { \alpha }<x\leq 1 $. We denote by $ \underline{M} = K( \alpha, \beta) $ the kneading sequence of $ T _{\alpha, \beta} $, by $ h( \alpha, \beta) $ its topological entropy and $ \Lambda = \Lambda_{\alpha, \beta} $ denotes its Lyapunov exponent. For a given kneading squence $ \underline{M} $ we consider isentropes (or equi-topological entropy, or equi-kneading curves), $ ( \alpha, \Psi_{ \underline{M}}( \alpha)) $ such that $ K( \alpha, \Psi_{ \underline{M}}( \alpha)) = \underline{M} $. On these curves the topological entropy $ h( \alpha, \Psi_{ \underline{M}}( \alpha)) $ is constant.
We show that $ \Psi_{ \underline{M}}'( \alpha) $ exists and the Lyapunov exponent $ \Lambda_{\alpha, \beta} $ can be expressed by using the slope of the tangent to the isentrope. Since this latter can be computed by considering partial derivatives of an auxiliary function $ \Theta_{ \underline{M}} $, a series depending on the kneading sequence which converges at an exponential rate, this provides an efficient new method of finding the value of the Lyapunov exponent of these maps.
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